physical prowess declines after your body peaks at 18-22 years of age (as does your fluid intelligence). this is considered pretty common knowledge (though, apparently not). for sake of clarity, let's assume we're all talking about your "explosiveness", or "fast-twitch muscle action", as that seems to be genetically determined and exhibits obvious age-related declines. slow-twitch muscles are not as affected by aging, and can be more easily improved through training.

performance in competitive sports will peak a bit later, as athletes become more experienced over time (just as your crystalized intelligence increases as you age). thus, it's the balance between physical decline and increased experience that sets the "peak" for a given sport. for example, in basketball, most players "peak" at their mid-late 20's. but most players are not as athletic at 28 as they were at 18. MJ, Kobe, etc, all peaked in their late 20's in terms of winning championships, yet how often do they take it to the rim and dunk later in their careers?

for me, i peaked physically at age 17 while playing sports in highschool. as soon as i turned 22, it was a night and day difference in terms of playing sports. in my case, there were lots of confounding variables such as lifestyle (smoked, drank, gained weight, didn't exercise, was often sleep deprived, etc during college), but even as i try to get myself back into shape, i'll never get back the explosiveness i had at 17 (even though it wasn't very much to begin with). btw, i'm 26 now.