The most formatting event in everyone life nowadays, and the one most negatively impactful is being born poor, that is age zero.

Young enough to sleep with your teacher

I grew up in a hoarder house and I'm pretty sure I ruined my life by teaching myself to do the dishes as a kid. I was expected to keep the place clean for the rest of my childhood, and that turned into me being the only one to do chores of any kind. I was actually guilt tripped into skipping college so I'd be available to drive my sister to classes.

So yeah forget all the drugs and murder and shit. The real worst thing a child can do is wash a dish.

Growing up in a house with sa at any age. For me I raw dogged life for my adult years blocking it out but stuff catches up and can be catastrophic.

Having success in sports, entertainment or anything that pays out real money. There's not a lot of happy stories about child stars.

If, when you're born, you don't choose to be rich, that messes you up forever.

I was watching a Netflix documtary about killers, the guy said he was on drugs by age 9...so pretty sure that messed up his life before the murder. It is debatable on if surroundings or self choices are why you try drugs I guess.

Being conceived by shitty people is more than enough to have a life arguably ruined permanently before it even began.

So much can happen to ruin a person's life at any stage, even pre-conception.

Any significant injury that results in paralysis would be a potential candidate. These become possible as soon as you become old enough to climb things like trees.

You're never too young to die, so the saying goes.

Playing with fireworks, firearms, or chemicals can be a solid way for anyone of any age to ruin their life.

Being born homeless.

I remember as a kid, I met these other kids that would drink from the side of the street. That sewer water was full of nasty including gas and oil runoff from the roads and who knows what else.

I also had an ex that would drink the chemicals under the sink as a tiny kid.

Be born a woman in Afghanistan... come out the womb to "And that's when she knew she had fucked up."

I'm not certain if this is in the spirit of your question but even a murderer in the US has privileges denied to a lot of other people.

Actually, the younger you fuck up, the worse are the consequences. A 13 year old go in a school fight, he is kicked out of school, has to go in another school further away, sleep less, see his grade fall down, and next year he'll be pushed to start an apprenticeship rather than high school.

A 31 one year old (otherwise a good citizen) does the same. He'll spend a night in police custody and at worst pays a fine (with a high probability that change are dropped because judges and prosecutor are busy)

Drugs at any age. Sure you can get clean but it takes a very pacific type of person to do that and there are more people who have failed then ones who have succeeded. I made the mistake of using drugs at a young age and it has destroyed my life beyond repair. If there are any young people reading this just remember it's not worth it. Trust me nobody hates drugs more then drugheads.

what can a ten year old do(besides mass murder & accidents) that messes up their life so badly that it is unrecoverable?

kill their own parents (it's not technically mass murder until you've killed three people).

Whatever the country's age of criminal conviction (typically 18). It's hard enough to get a job WITHOUT a criminal record.

Depending on the country and/or crime, a juvenile record will also prevent being able to work in certain jobs/industries.