There are rules about how many mandates you have after a certain number of wins, at least, and years on tour. I'd have to look it up, but I know Fed has reached some level, as has Nadal. Not sure if age, per se is one.
Yes, but beside those, there is the one that I mentioned, that is once the player is 31 he can chose which and how many tournaments he will play, nothing is compulsory for him, neither there is set number of tournaments they have to play in a year. That rule applies now to Rafa from today because he just turned 31.
1.08 Reduction of ATP World Tour Masters 1000 Commitment
A. A player’s number of ATP World Tour Masters 1000 commitment tournaments shall be reduced by one (1) tournament for reaching each of the following milestones:
1) 600 matches (as of 1 January of the commitment year);
2) 12 years of service;
3) 31 years of age (as of 1 January of the commitment year).
If all three (3) conditions are met then the player has a complete exemption from the ATP World Tour Masters 1000 player commitment.