In my personal opinion, the key to training is having a cycle pattern where your main trainings slowly turn into your secondary trainees (Or just not the main focus). This allows them to still get pops that help them but allows you to develope younger players at the same time. This might help you as well:
Here's a list of training speed for each age:
18 100.0%
19 95.0%
20 88.0%
21 78.0%
22 70.0%
23 60.0%
24 51.0%
25 42.0%
26 35.0%
27 27.0%
28 21.0%
29 16.0%
30 11.0%
31 7.0%
32 5.0%
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs