18y are good for investing, as the cost of using one is close to nil or negative.

Even for non-fast trainers they train reasonably fast that their market value will increase a lot for lower levels. After a few seasons the market value can be a few times more than the original cost plus the wages paid during the seasons if trained regularly, meaning you can gain money by buying young players and selling them before their contract expire.

This is true for most players under 24. If you can't sell or an old player is retiring you will lose the money you have invested on him innitially. If you lack money it is a good idea to avoid players above 29 because it is difficult if not impossible to resell so the money for it would probably be lost once you add him to your team.