Hey I'm thinking about upgrading my training center to level 4 but before doing so I read that the older the player is the less effect the fixed training will do. Have anyone tried to train older players? Perhaps older than 26-28 years old. Does fixed attribute training affect the player's star rating? If you trained your older player's fixed attributes, how much did it improve by? Thanks.

Almost all the features are affected by the age factor (%age)
http://rockingsoccer.com/en/soccer/help/home/category-2/article-44

Trainable features: Blocking (slighty affected), Dueling, Passing and Scoring.
Fixed features: Power, Speed and Endurance (affected twice).

The two Features that aren't affected by the age are: Talent and Tactics.

If you have a 24-26yr player, he may have a 90%-95% age factor. So, if you train him in Power or Speed, he will gain a 0,90-0,95 ball in that feature. If you train him in Endurance, then he will gain 0,81-0,90 ball.

Yes, fixed attributes also affects player's star rating: http://rockingsoccer.com/en/soccer/help/home/category-2/article-46