Well training slows down a little bit every year. So a 19 year old is slower than an 18 year old. And a 22 year old is slower than a 21 year old. Where you stop training is up to you. If you want to make a national team player you will have to keep training after 22.
A level 4 coach will train the same for level 1 to level 2 as if would for level 19 to level 20.
However, there are other variables that might cause the speed to change.
If the skill you are training is a lot higher than other related skills then it will train slower.
If the skill you are training is a lot lower than other related skills then it will train faster.
Chances are that a player will be older when training from 15 to 16 than from 7 to 8 and so it will take longer due to the age.