Realistically you have to start sprinting as a child. If you don’t, it’s not that you don’t get near your potential, it’s more than your potential changes. Sprinting is more about central nervous system conditioning than it is about fitness. Your CNS is shaped nd responds the most flexibly during childhood and teenage years. This is why you can get a fatass in their 40s who played a load of soccer in their youth maybe run a 70 second wuarter after years of doing nothing and beat that guy who has been running 3 hour marathons for the last 15 years. The mildly obese former soccer player might beat the marathon guy by a couple of seconds but he will be winded for 5+ minutes while the marathon guy would be ready to do it again after about a minute.
Sprinters and gymnasts all start early. This goes for anything that involves explosive conditioning. You bet every fast sprinter had a super active childhood even if they weren’t actually training for speed, pretty much every great athlete was doing something in childhood to stimulate the development of their world class skill. Usain likes to play soccer, I could see him playing a lot of that as a child. Maybe if he didn’t he would be nowhere near as fas and the only genetic advantage he has is a super stimulated/easily activated central nervous system and if he waited until adulthood to activate those speed genes them they wouldn’t be nearly as effective and he runs 12.58 instead? Usain is obviously a genetic outlier, I believe he has always had the genesto get faster than most no matter what but you have to ask: what did he DO to get make those genes work that way?
if he started in his 20s then how good he would get would depend on how he treated his body during puberty. Is he just living an average life in this scenario eg: eating a normal amount of food but not really exercising, one of the band kids or is did he live like an extreme binge eater who is now seriously obese? Early lifestyle shapes your muscular stature and central nervous system, if he started in his 20s and was completely sedentary up until that point he would have nowhere near as much advantage as he has now and would probably be mediocre.