Sometimes yes, sometimes no.
For example I am in my Schalke save and I am selling Draxler in the summer after 1st season.
Reasons? His fitness was always under 80% 20mins into a match... and 36 million $ is good for his age
I buy exceptional young players, keep them for almost their entire careers and sell or release them when they reach 31 or 32. And if I'm confident that I have better players coming through, then I'll accept or negotiate offers for those in their late 20s or early 30s.
Depends on the player. If they continue to preform consistently I keep them until they retire even if the play fewer games towards the end of their careers, if their stats drop rapidily once they hit 30 then they have to go.
GK, CBs and DMCs i keep until retire cos their mental stats raise and i need those in the prior positions.
MCs, WBs and AMCs i keep until they run out of pace. Then 2 options, retrain as DMC (Lham in my save with 35 ), or i sell.
Also, not loyal with foreigns. I've bought Januzaj for 3.5, now playing for the B-team he is valued in 7,5. next year he will be brought to the main squad as a reserve and will be sold for 15 Mill or more.