I have actually heard that your raw muscular strength peaks after 30.
I think 90% of the decline is psychological.
First of all you're taught that your skills will decline. Self fulfilling prophecy.
Second of all the mental drive and motivation kind of go away.
Third of all, (and this might be the same thing as #2) you have different priorities in life. I'm only 28 but in the past 2 years I have mellowed out considerably. I don't care if my bf is in the single digits or if I can lift a lot of weight. I mean it would be nice but there are so many other things to life.
Honestly, I think around mid-30s is when you start to lose your edge at the elite athletic level, like WODIN said. Smart athletes make up for it with the mental. That's why you see lots of 40 year old QBs but not many 40year old WRs. Also injury recovery goes down just because your cells get tired.
One thing I don't really get is there are some fucking GEEZERs in the NHL. Yzerman, Lemieux.
For people that are seriously into livign a healthy lifestyle, I think it's entirely possible for a 40-year-old you to lift and run circles around a 30-year-old you, assuming you haev been practicing just as hard. Hell if you were a serious basketball player, I bet you could go back in time and beat your 30-year-old self at one-on-one.