I think that depends on the number of games they play during the season before.
I bought 1 player with 30, 2 with 31, other with 33, in diferent seasons, and all will retire tomorrow with 34, they all were playing in first Team, so many games per season.
In other Team a player with 30 years old was received in level 1, almost Never played, next season he retires with 31.
On other Team a GK with 31 only played the first games, because After that i tanked, at the end of this season he will retire with 32.
I also have other exemples of players with more that 30 years old, that play with regularity and still not retired.

This is only based on what i have Been seeing in my 3 teams, so i cant be absolutly sure about it