Every single strong GM who can make a living off playing in tournaments has started pretty young.
You asked "how adults even play this game" - they just play normally, and rather imperfectly, like the vast majority of chess players who play for fun. You don't have to be a GM, or any master, to play and enjoy the game.
I've learnt recent at my age I am out range to reach top level. The fact is one must begin asa child to have this option. How adults even play this game?