Here is another example. Me and A-Rod are about the same age. When we were both 18 years old who was a better player? A-Rod, of course. He was the 1st pick in the draft when he was 18, everybody heard of him. Who heard of me when I was 18? Nobody.
Now that A-Rod is 32 and has 600+ HR he has improved alot as a pro athlete. But he really didn't improve that much from the time he was 18, because he was already a great player at age 18. By the time he was 21 he was already hitting .350 with 36 HR in the Majors, so he really hasn't improved that much in the past 11 years.
Me, on the other hand, had I been able to get a minor league contract at 18, and then called up to the big leagues by 2009 would I be hitting .350 with 50 HR per year? No. But would .270 AVG 23 HR & 86 RBI be possible? Yes, and that would make me good enough to be an MLB player. So in the past 14 years I would have improved drastically as a player, even more than A-Rod, but I didn't get that chance.
Also, alot of being a pro athlete has to do with how quickly you physically mature. If you mature much slower and don't really hit your physical peak until you are 25, and aren't even physically big and strong enough to play by the time you are 18, you are not making it. Beacause by the time you are 18, you have to start thinking about how you can make money and be successful in life. If you are not on a team, getting scholarships etc., you cannot rely upon sports to be successful in life, because sports isn't producing anything for you. When most pro athletes were 18, they were already getting scholarships, some contracts, perks from their respective schools, the ability to travel and play, equipment, coaching advice, etc.., so sports was already giving them something in life. If you are not on a team, sports isn't giving you anything so you cannot make the time and effort to be a better player, because life doesn't give you the opportunity to. But whether or not you made your HS or college team was not your decision, it was the coaches. And if they don't pick you, you can't put the time and effort in, mostly because of financial reasons, to be a pro athlete.