I want to put a point from another game i play www.sokker.org its similar to hattrick, but has many things better than hattrick, and one of them is that each player on the game has a TALENT. On hattrick a player with 17y old increases after 7 weeks of training in a skill, on sokker a player can take 4 to 8 weeks, depending on the TALENT of that player, if he has 17y, so u can have a player with 20y increasing after 5 weeks of training and a player with 17y taking 8 weeks, so age is not that important, talent is more important.
This talent thing makes the game more close to the real thing, where each player develops at one speed, no mater his age, of course that younger ones can develop faster than old dogs :-)