For me I would get rid of the ones that you DEFINITELY know will not make the grade for getting into the first team. Keep the ones you think that have potential and if you get any offers from other teams to take any on loan then you can take that option as well. That way they are getting regular match time which is important as we know.
The ones you want rid of, change their Squad Status to Not Needed By The Club and set the Transfer Status to Transfer Listed and Loan Status to Available For Loan (if a team wants a player you want rid of on loan, you can give them the option to buy him at the end of the loan) . Also for these ones as well (during the transfer window) click on Offer To Clubs to see if you get anything back. Ones you want to keep and think have possible potential, you can set their Loan Status to Available For Loan and leave the Transfer Status to No Status Set. That way if you get any loan offers for these players, you can let them go out on loan and that way they will at least be getting match time which is always handy like I say.
For your Under 18s, then the only thing I would do is if you get any offers for any of them, whether they are for loan or to buy, then just consider the player and what your back room staff say about him. If they have decent potential and you won't leave your Under 18s short then you can loan them out as at least they are, again, getting regular match time. If you don't think they are good enough and you get a transfer offer then, again, take it as long as you don't leave your youth squad thin on the ground. I have done this before and shot myself in the foot when injuries started happening.
It's all a juggling act.