Better time to sell?

Ok so today is almost gone but for the future. What do you say is the best time to sell?
Lets use exaple of Level 1 but it surely similar for next levels.

Lets say I have 24Q young good player on my squad. Sell him now or wait till day 1 and sell him to lower level guys?

I'm selling now because next season my players will be 1 year older so they will lose on value, how much, don't know.
Important is to sell 31y.o. players, because next season they could retire (play last season) and then you can't get money from selling.