Two things.

1. And the most important one. My guitar teacher taught me that. Whenever I feel pain in my wrists, hands or fingers I stop, no matter what. I take a break, see how it goes afterwards and if it's not better I stop doing what I did for the rest of the day. Of course sometimes it's important work stuff that needs to be done. As for me, most of the times it's my right hand that hurts from mouse useage, I switch sides and get urgent things done, but still stop afterwards.
2. Sports, as usually, building up muscles in my hands and arms helps me personally. Not sure if there is scientific evidence for that.

But yea, number 1 is the most important thing, no job is worth having pain for the rest of your life.

I think the thing that helped my wrist paint the best is getting a split keyboard. I also ended up learning how to mouse with my left hand, and for me that’s helped, too. (And actually with how I set up my layout on my keyboard makes me a little more productive.

Ergo keyboard and vertical mouse.

I'm currently using the Logitech K860 and the Logitech MX Vertical. Easy rec - cost effective, easy to get comfortable with as opposed to the more intense/expensive options.

Made a huge difference for me.

I had been struggling with severe RSI for a few years and no one thing helped. I would try something out and the pain would return in a few weeks. What eventually completely solved my problem is variation. I have several working spots using different devices (traditional mouse, vertical mouse, thumb balls, trackballs, pen tables, touchscreens). I've made sure to just change posture and devices every few weeks. Ever since doing that, my problems have completely gone away. A mobile standing desk that you can adjust for squatting to slouching to sittin to standing and walking is great adds a ton of variation.

I had a scare a while back with numbness in my pinkies and ring fingers. Did some research and that's ulnar nerve issues, not carpal tunnel. It's a big nerve that goes all the way from your shoulder to your pinky/ring finger, so I addressed all the places it could be getting pinched.

Chest: Split keyboard helped me a TON here. I keep gaining weight as I get older and realized holding my hands close enough together to type on a normal keyboard requires constant tension in my chest. Split keyboard not only relieves that tension but makes it easier for me to keep my shoulders rolled back. I read all about the ergodox and the benefits of columnar layouts etc. etc. Bought an ergodox and work was too busy to take the time to relearn typing on it, so I got a kinesis freestyle edge RGB which is just a standard keyboard that's split. Is it as good for you as the fancier options? No. But did it solve my chest tension issues with zero downtime to relearn typing on a new layout? Yes.

Elbow: Invest in a good chair, with good adjustable armrests.

Wrist: Got an MX Vertical mouse and have been very happy with it. Took me a little bit to get used to, but after a day it feels pretty natural. This keeps you from twisting your forearm, and kept me from wresting the corner of my wrist on my desk. I keep a vertical mouse and a standard mouse on my desk, standard for gaming and vertical for everything else. If you don't want to spend the $$ on the logitech, Anker makes a vertical mouse that's like $25 or something.

Re-iterating what a lot of other people said: Just listen to your body and don't ignore stuff. Not a Dr. but I've heard a lot that this is the kind of stuff that if you respond to it as soon as it starts you can stop it from progressing, but if you ignore it for months it can become permanent damage that surgery may or may not even help with.

I do these stretches that Day9 talked about very regularly. I used to play some Starcraft 2 but the habit stuck.

A huge one for me is the desk. Height adjustable chairs are standard. Height adjustable desks need to be also - you are unlikely to be one of the rare humans with exactly the right torso/arm size for the standard desk height.

The standard height was normalised when everyone wrote with a pen - it's just too high for typing even for me and I'm very tall. Look up the proper arm/wrist/head angles, you'll see it's not possible to achieve unless your feet are on a tall footrest and your chair goes higher than the standard office chair height. It's far more practical to lower the desk than raise your feet/butt.

Quality electric height adjustable desks are affordable these days, so aside from that benefit you can also spend some of your time standing up (don't stand all day - also start with short stints and build up to longer ones).

Monitor height is important too. I've never found a monitor that comes with a stand that is high high enough for me and laptops are obviously way too low. VESA arms are best - but books work too. The main advantage for VESA over a stack of books is you can put other stuff where the books are.

I personally don't think ergonomic keyboard/mice are the way to go. They tend to force you to keep your arms/hands in exactly the same position. It might be a comfortable position but it's still no good to repeat exactly the same muscle movements over and over.

Instead, go with a (good) regular keyboard, maybe a split one but not a sculpted one. Your hands should "float" while typing with just your finger tips touching the keys, don't let your palms touch anything - your arms should be moving while typing not fixed in one spot. It will take time to get used to this, but it's worth it.

For the mouse... learn as many keyboard shortcuts as you can and get yourself a desktop trackpad, put it where laptop trackpads are, for scrolling, gestures, etc. You won't have RSI issues form mouse use if all you do is click on things where you actually need to do that.

If you're going to a doctor... be careful. Most of them really don't know anything about this issue and unless they're a friend or family they likely won't be willing to spend the time necessary to research it just for you. I'm not saying don't go to a doctor but make sure you find a specialist and verify anything they tell you. I've received some really bad advice from doctors.

I mainly use ergonomics. I got a chair that works for me, I switched to a Kinesis Freestyle Pro, I switched to a trackball mouse, I raised my monitors up, and if I'm going to be doing a bunch of typing, I throw on wrist braces with aluminum stays.

The split keyboard and better posture were the biggest wins for me. The whole mechanical keyboard fad has been terrible for people's wrists.

Other things I've heard worked for some people that are worth trying: warm gloves or compression gloves. Apparently some people get pain in their wrists from typing in cold environments (like an office). Taking breaks, doing wrist stretches, and having hobbies that use your hands differently can certainly help.

Is there anyone with RSI or a carpal tunnel? If so, at what age did it start to develop, and does it comes suddenly or is there a warning you may miss or ignore?

I've spent most of my life in front of a PC. Since most of my hobbies are on a computer, and I work at a computer with which I started at a very young age (basically ever since I got a dreamcast when I was 4), and aside from also thankfully being forced to do sports at least once a week for most of my life (which I unfortunately stopped a few years ago, apart from ballroom dance from time to time - and I'm 27 now), I'm literally almost all the time in front of a PC. I never had problems with any kind of pain regarding wrists or hands, but from what I've heard carpal and RSI is something that can come pretty suddenly and it's not a good time. Is that true and should I start worrying, or will I probably notice when the time comes?

I am 42 and I have been dealing with this for the last few years. It comes and goes but when it comes it's hard to get any work done. My best advice is to stop working when the pain starts, sit in a proper office chair the right way straight up do not slouch, take frequent breaks, do RSI warmup stretches, and finally get a personal trainer to get the blood flowing in the arms. I do all of these things and even have switched keyboard layouts. I current use whisper c++ to dictate my slack convos and work emails. As for the mouse use I have tennis elbow from mousing I have not figured out a way yet to stop that, I'm going to try mouseing with my left hand and see if that is doable.

See a doctor in sports medicine, physical therapist, or occupational therapist. Basically someone with a medial background specializing in the musculoskeletal system.

I while back I had an issue with tennis elbow and numbness in my hand. I went to a doctor specializing in orthopedics and sports medicine. First visit was an exam and ultrasound of my elbow. He also asked things like how I slept, and if I was sleeping on my arm. There was some swelling and inflammation, but noting too major. He referred me to a physical therapist.

I saw the physical therapist for a few weeks. It was mostly teaching me how to do certain stretching exercises and strengthening the muscles in the area. There were some massages with a muscle scraper. That was weird and not pleasant at first, but did help.

I dislike having to disrupt my computer activities every hour

Nobody likes being taken out of the zen that is being in a flow state, but humans evolved to walk and move around a lot. Take breaks and go for short walks. If I were to give advice to my younger self, it would be to take up running. Humans and a few of our domesticated species are the only ones that can run for long distances. Some members of the species enjoy running for long distances and do it recreationally. They often say they enter a flow state while running.

I don't have a source for the efficacy, but I've been doing rice bucket training, which seems to help, at least in the short term. I think the idea is that RSI is often caused by or exacerbated by muscle imbalances in the hand, so working the muscles you don't use for computer work can alleviate and prevent problems.

I go to the gym 3 times a week to prevent any kind of injuries like back pains, obesity or carpal tunnel. Before I started I was nearing the "severe obesity" bmi value and had constant back pains. Getting fitter decreases risk of illness and injuries.

As for the repetitive motions. I have ADHD so I'm always moving some parts of my body. :/

A lot of people are talking about better ergonomics, which is great and helpful for preventing symptoms. But for me, what helped most to reverse my symptoms was performing finger extension exercises throughout the day.

Mouse clicking is all finger flexion so you need to balance that out by working the opposite muscles. Finger extension stretches (lifting one finger at a time off a table; stretches the flexor muscles), finger tendon glide, and finger spreads (spreading fingers apart with a resistance band; strengthens extensor muscles; my fav) have all been incredibly helpful in managing symptoms.

Ball squeezes on the other hand aggravate my symptoms because they work the same muscles and motions that clicking does. So don't do those.

I switched to Dvorak keyboard layout almost 10 years ago and it stopped the growing pain on my wrists. Might worth a try. No need to change your actual keyboard, just set your keyboard layout settings in your operating system to Dvorak.