There's a decent gap between GM and world champion to be honest.

Absolutely, a huge one.

But still, chess is a game that favours young people that have more energy and can calculate more, and that peak is achieved in one's late teens.

But also a big gap between 12 and 18, so who knows

Those statistics blow my mind. For reference, Bobby Fischer became a GM at fifteen.

Online and computer chess have changed things. 12 year old kids generally can't travel to tournaments, but they can play against other strong players or against the computer online.

Fischer lived in New York, and therefore could play in the Manhattan Chess Club.

Yes, they amass thousands upon thousands of games at a very young age. I did the same programming my C64 in 8th-10th grades. The hours just fly by, doing what you love.

One other factor is that 3500-level chess engines are freely available for anyone with the net to analyze every situation, every move.

And then there are the streamers like Hikaru who teach chess so brilliantly. He is a true one-off, to be that top-level and able to live-comment his own blitz games. It is an underappreciated and completely unique talent, and enlightening for the chess afficianado.

Yes, they didn't know this back then that it was possible. If they had known, they would have certainly made Fischer a GM much much sooner

There's also a decent amount of controversy around really young GMs. Basically that their parents game the system by choosing official tournaments with burnt out GMs with low ratings so they can get their norms easier. Mishra recently had a lot of backlash from top GMs with those types of accusations. If that's true, those players will likely never reach the top ranks, but who knows.

I've seen the same suggested of Sergey Karjakin, and he made it to the top (and I've seen it suggested that it helped him get to the top, that being a GM sooner got him more access to top trainers sooner).