Like Skidz said, generally i'd sell the free youngster.

I've never bought the premium youth academy player myself, as usually i'm savvy enough with the TM to purchase better players for the same amount, if not fewer, of tokens.

Generally, the best youth academy players cost 21+ tokens anyway, so at 14 tokens he isn't the fastest trainer anyway.

I myself, would look at the transfer market, but it's all down to whether you can train the academy player enough to make him worth 14 tokens.