Actually that's a good point, I had overlooked the fact that Federer can cherry pick the tournaments he wants to play in.
A bit of a disadvantage here for Nadal. Maybe the rule should not be based on age, but by the number of matches played on tour.
The rule is not only based age, but also on number of matches played, AND number of years on the tour.
These three conditions must be fulfilled before you have an exemption.