Forgot to add... one thing about age in tennis is it's not entirely a like-for-like comparison. Some people turn pro younger or mature younger. In the late 80s and through the 90s there were dozens of teens in the top 100. In fact, sometimes as many at 5 or more in the top 20.... Nowdays - there isn't a single teenager in the top 200 (men's). Modern strings, training and intensity required to be a top player have almost negated the likelihood of a teen beating someone in their 20s as used to happen a lot in the 80s when a talented newby could literally have a good day and blow a top guy off the court: Becker winning Wimbledon, Chang the French Open, Sampras the US Open etc..
If you look at a tennis career as having a prime of 5-7 years you could consider the 'prime' for top guys as from when they won their first slam.
Nadal won 10 slams in the 7 years from when he won his first. Federer won 15. The slow-down later in a career is the thing which is hard to avoid. Some are athletic 1-in-a-billion people like Federer.... Will Nadal be one or will his harder career have a toll on him earlier?
Federer being 1 step slow means he loses a few % more of his close matches because he has so much more innate ability to win points. Nadal being 1 step slow means he loses 20% more of his close matches (David Ferrer at the Aussie Open, Del Potro at the USO in 2009, Tsonga at Aussie in 200

... so dependent on speed is his whole game. (which is why he's been trying to hit-out more in the past 6 months - to learn to end points quicker - surely so he's not a wounded antelope once he loses some of his court speed in a few years)