It absolutely is older than that. I ran a tiny ~~alpha~~ server for three people on my parents' old desktop computer back in 2007.

Edit: alpha isn't the earliest, there was indev and infdev. Can't remember which one it was that I did a server for. It was before notch even had made plans to sell it as a game and he was regularly posting threads on 4chan's /v/ideogames board about it. Water didn't even have physics at that point. No real mobs, but you could spawn in a "frog girl" that was way too detailed and obviously a placeholder. Just bare minimum procgen world, dirt, stone, "grass" (on dirt blocks), "water", and trees (trunk and leaf blocks). No lighting engine either at that time.

Still makes me laugh that a friend who I invited to the server who never took me up on it wanted to tell me about this awesome new game called minecraft a number of years later. You mean that game I tried to show you years ago?

Anyway, for 15 years are we just counting after it left beta? Because it was around for years before that, and the general consensus was that the "1.0" release was just hyping sales. It was still a mess back then.