For me is not such a paradox, and I will explain you why.
I am along time observer (I mean decades here) of the performance in track and field. That was my first favorite sport as a child. And by what I have seen for that duration (since 1970's) that:
1. The speed (explosiveness) is maximum at 25y of age, then it deteriorates;
2. The endurance (resistance) still develops until late twenties and can be maintained over 30y. This is because the distance runners are capable to still get better in those years.
So the physicality what you mention is rather pertinent to the endurance of the athletes and not the speed. My point is that a player can be better in his late twenties than in his early twenties.
Never mind that mentally will grow as well. Wawrinka is a quintessential example of that, he is strong and he became mentally a different player under Magnis Norman. Stan plays better later in the events (see the 3 slams he won) than in the beginning, thus there is no fatigue factor as far as he is concerned, despite his age.
