All skills will train more slowly in older players. For example if passing takes 2 weeks to pop in an 18 year old it may take 4 weeks in a 24 year old and 6 weeks for a 28 year old (numbers made up). The pay back is that you get to play a better player in your league game and that the return is better, as it is better to get one pop on your best player than two pops on your worst.