I contributed to a browser engine around that age (KHTML, which later became WebKit and Blink), and while I don't work in browsers right now, much of that knowledge, mindset and of course the professional network have done much to shape my life. And a fairly successful career, for that matter.
If teenagers could make small contributions to LLMs via open source then sure, go for it. Optimizing llama.cpp or similar would be a good learning project that might later get you good work via social networks. Contributing to open source is how I got started too.
Unfortunately, training LLMs isn't something that fits well to open source open collaboration. Inferencing codebases are better.
In that sense it's more a "seek out the open source community and real projects when young" rather than "do web browsers", with a bit of "look for ambitious types of projects few get to work on".
A lot of people will make better decisions with a few yeas more maturity, and spending a few years developing themselves.
University will help a lot of people, and for some it will help.
There is a lot more to life than making a living.
The early Internet (which the GP mentioned) didn’t even have the web. But that’s nitpicking.
Paradoxically I coded way more between ages 12-14, I regret my wasted late teens.
ages 8-15: lots of great 8-bit computer fun.
ages 15-20: girls, booze, motorcycles.
20 onwards: get a PC, back to computers, realize how much I've been missing.
To be honest, judging by my own kids and their friends, late teens seem to generally be an era of hard to avoid stupidity.