Academy player is good for subs on the first two years. Even a 1* rarely gets a form below 7. In fact, I rely heavily on them as reserved for important games, and have quite a number of cases where they took the MoM with a rating of 9 or 8, despite being the lowest quality player on the field.

If you keep free youth for at least 2 years, they can also lower your average quality which is an advantage in league and cup draws.
If you train them with the intention to sell them through negotiation, the free youth could also be sold for tokens, which is best done during age 18~20 when they are younger than scouts. You will find no problem selling them for at least 1~2 tokens each if you managed to have them listed in negotiation with a special ability.

The token costing youth is always a fast trainer, but it is only worth it to sign early in the season. Late comers are more expansive in tokens and you got less time to train or get him into play before he ages.
Main advantage of the token youth is that he cost no money to sign.