They are usually replaced with new old 3* players close that retire. At least that happened at one of my passive played teams.
On my server there are some players that are 33 or 34 years old.
Furthermore in lower levels it is not dumb to sign players that are 26-28 since they are cheap and one can save money to build up the infrastructure. In the 3rd season (28-30 years old) one usually finds a buyer who let them play up to the end of their career.