If I remember right Arthur Dake (1930's player) did not pick up chess until he was 16 or 17 and he later became a grandmaster, but I think he was an exception. I think you're right-most strong players were strong before adulthood and later development added frosting to the chess cake. The young brain is probably more flexible and better at memory retention, plus you have time for chess play and study which is mostly lacking once one reaches adulthood. Nevertheless, one can improve some even if one is an adult as long as a defeatist attitude is not present.
Chess Ability Correlation

You have to also factor in that kids start to worry about school / grades / future a whole lot more in their teens ... and as they transition to adults, a paycheck.
I've seen a talented highschooler at the club give up the game because his girlfriend didn't want him spending time away from her. I've seen gifted 15-16 year olds drop chess altogether because of grander college plans.
I'd think that in today's world, a 4-8 year old chess prodigy leads a rockstar existence, his entourage (mom +dad) take him all over the country, stay in fancy hotels ... they pay the bills and get him coached by the best ... all he needs to do is study + perform. This "free from distraction, concentrated effort" should account for a rating jump, assuming the kid has potential for the game.
Hi all,
I'm about a 1300-1500 USCF rated player. I started playing the game when I first noticed the pieces at my local library. I was probably in the 4th grade??? I quickly left the game for most of my adolescent life after only learning how the pieces moved and never took it seriously until college. I believe my first rating of around 1300 at a USCF tournament correlates with when I first picked up the game of chess. And I see many examples of this --- The younger you started, the higher your rating,etc. I think I read somewhere that most if not all Grandmasters had very high ratings prior to adulthood. So, what I'm trying to say is that there seems to be a correlation between WHEN you picked up the game and your rating. What do you guys think?