Yes, the gaining ability last about in the same level from 18-21 y.o.
Then, it's from 22-24 y.o. and can last in some good levels from 25-27 y.o.
I took this picture just know

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all the 5* players have Q9 quality.

the players in the red box are ex-fast trainers but as you can see they still can have good gaining numbers so if you have greens, it's worthy to give them a 3bars/practice 2-3 times/day.
Another and maybe the most important thing is to making the plans of your team. Not only for this season but for the 2-3 next.
It's the strategy and the tactics
So, to know which players to keep and for how long (after checking their training ability and performance of course).
Those players in the red box, i 'm going to sell them in the end of the season -well, most of them.
The player in the green box, SANTOS, I 'm saving his sp, thinking that maybe I 'll give him also a DMC position if not find any DC/DMC next season.
The players in the orange box, I 'm keeping them to under-level my team so after 2 seasons (in level 12) to go for my 3rd treble
I give gaining sp in special ability which doesn't level up their quality. I 'm gonna add two more of them.
This season I 'm gonna win the league. In Champions League, there are 4-5 teams that have 5-6 6* players. Some of them are scout/list players they have bought but the good with my team is that it's still earning sp.
I don't sign those 640 sp I have because I want to keep a low profile to them.
I can move on with that quality and if the moment 'll come, I 'll give those points to my players.