There are 3 things related with market value, age, quality and training speed.

What you are saying is that you whant to pay more for a 30 years old player that for a 18 years old one, because If the market value did not decreased with the age that what would happen.
Do you really whant to pay the same amount of money for a player with 21 years old with the same quality and speed of training of one with 18 years old?