After 30 years old, any player can retire, i have seen that the age of retiring depends of the number of games played the season before.
At the beggining of this season 4 of my players retired with 34 years old, the maximum. I bought them with 30, 31,31,33 and all were playing regular in m first 11. And i already had more exemples.
On another Team i had a player with 30 that almost Never played, next season he anounced that will retire at the end of season.

With 30 years old it is strange not being able to sell, i can only think If he has a Buying offer from negotiations.