This is mainly going to come down to what style of manager you are and what type of token budget you have. There is 2 reasons why I could think why a younger player aged 18-19 is of any great value, the first one is someone looking to keep them in the squad for a number of seasons due to good natural growth from training/playing games and the second one is Power Training and we know that PT improves quality in training over a space of a few days.

Other than that your right, Its easier just to save your tokens and buy less attractive players in the mid 20's to about 31 years old without special ability that can be easily picked up off the auction for 1-2T each and then these players arent strong enough for you next season and you just list them because you can go back to the market again for another squad of the same type of players you nhad previously for the same price. This strategy is mainly best for players that don't buy tokens and operate on the minimum budget possible.

Then the other strategy is the scout players. The extra available ability at a premium cost and these players can last 3-4 seasons through natural training and buying at the beginning of the season.