The trainer level doesn't have an impact in the decrease of training speeds. Think of age as a multiplier after the level of the trainer is calculated. So if he's 18, he will receive 100% of his training, 19 he will get 90% and so on. But now, higher levels of trainers do not change the amount of training received as age is concerned.
Just gonna mention this, the difference in trainers from superior and up is very small. If I remember correctly superior is 97%, exceptional is 99%, and WR is 100%. Even advanced gives you 93%. Level 6/7 are generally recommended for NT training. While you can use them for club, they are cost inefficient.
2x NBBA Champion: S55, S56. 3x USA Cup Champion: S54, S55, S56. WR for longest home streak ever at 11 in Season 47.