Here is a fast trainer I have. When I got him, 18yo, he started with +120 - +140 (all the numbers are in 3 bars, practice).
He reached almost full 7* the first year. Next year "lost" a star and starting over.
Now he is 26, he is almost 6* as all those years I had him. Next season it will be difficult to reach the 6* but he can be useful still in his 5*.
Those are some gaining at 25 y.o. and at 26 y.o.
I 've seen players near 30 y.o. with 6-7* but it needs a lot of greens to keep them that way.