It all depends on what benefits the loan will bring to the player, you have to pick the right club.
I tend to only loan players out if they are going to be a key member of the team and played in a system which suits the player best. If they have only just turned 17 then I'd rather them stay in the youth team and be tutored until they mature before sending them out, unless of course they are exceptionally talented for their age.
As well as this I generally look to loan players out for a maximum of 6 months initially with the possiblilty of using the second half of the season for tutoring or finding a higher quality club for them to go to.
If they don't need any tutoring and already have a good personality i'll generally send them to a league one club when they are 17/18 if they still need tutoring they won't go until they get it, i also try to ensure that they have the minimum stats to be competent on the position they are going to play(enough pace and dribbling for a winger, good defending attributes for any defender,etc)