[Brian Swartz]

I'd suggest reading post #4 in this thread, where some absurdly long-winded blowhard waxed on about various things. There's other stuff throughout the thread as well of course. Here's the relevant part:

Originally Posted by blowhard who looks a lot like me

Age % -- This can be confused with aging factor but is not the same thing. This describes where a player is in their development, and is a multiplier that determines their actual ability at the time. For example, Mooljee has a raw ability of a little over 2.0 in skill, but because he's a young player, age % is only 68% and so he plays at a level of 1.4 instead. Skill, service, speed, strength, and endurance(twice, i.e. multiplied by 68% or whatever twice instead of once) are all affected by Age %, but the other abilities(mentality, doubles, talent, etc.) are not.