players depreciation - best time to sell

i am a level 6 player soon to be level 7 in the coming days....i inherited an 18yo DC(41) recently from my youth set-up who in a week or so has trained up to (43).
with my team average being just over 50, he is just falling short of the level i'm requiring.....but when would be the best time to sell?
his value was £9.3m but after a week or so training his value has climbed to £10.6m....so in a few days he will become a 19yo & i'm assuming his value will drop, but considering his skill improvement & other factors, could anyone advise roughly what percentage of a players value is lost when ageing in relation to players value when improving?

value will drop about 10% when he turns 19. Some are more, some are less. I would train him to 1 skill point from the next star so he can be placed on the transfer list and then leave him there. Or sell him via auction on day 1 of the new season during the afternoon/evening (your time).

Here are picts of my squad on level 5 and the next day when they became level 6 so you can see the change in player values.

Attachment 8824Attachment 8825

I heard that the players worth drops 15% at the start of the new season.

Put him up for sale on Day 1 and hope it starts a bidding war.

Players with special ability/young players with quality ending 4/9 i would sell them the first day of the new season, the others at the end of this season. Crazy bidding wars remember :)