The average level of the 19 yo's and 18 yo's is about the same.
is it smart to choose them ?
It depends on what you want to do with your player.
If you want to train them and make one your new team icon, then I would prefer a 19yo 4 stars skill and 4 stars potential over a 18 yo 5 star skill and 2 star potential.
But if you plan to sell your draftees then I would prefer 18yo high potential players over high skilled 19yo draftees.
But once again that's my opinion ;)