> "Becoming a GM" only seems possible for people who reached master strength by their early teens.

Mostly, I think, because becoming a GM is a full-time job and you don't earn any money while doing it.

You need to go to a lot of tournaments around the world, continuously study to get prepared for opponents, and have enough money to fund it all.

So, you really have to want it as an adult and have nothing else that you would prefer to do.