No hard rule on this. It totally depends on how well he performs, If he can still grow enough to remain productive (assuming you promote). It depends on if you are a token buyer or live on 35 plus some freebies.

At the lower levels money can be tight if you don't spend real money but as you get to higher levels you tend to have plenty. I have a 22 man roster with an avg age of just under 20. But that doesn't mean that i throw away older players automatically.

What I am doing is buying all teenage nordgens, training them up, and I plan to keep the squad together for as long as I possibly can. I want to not only have a huge trophy case but a hall of fame with players who started several hundred games together and a couple guys who scored 600+ goals in their career.

But I pay cash to make that happen. Not everyone can do that.

On a strict budget the best way to go is to is to buy cheap older players near 6* every season and compete the best you can.