Parroting the others here, but play them.
Age matters. To go a bit further with WJ's point, SI has said that playing competitive football becomes a bigger requirement for a player to develop at 18. Prior to that, aspects such as facilities, coaching, etc, have a bigger impact.
The best success I've had with young players has come from playing them. Every wonderkid I've developed on the last few versions has been thru giving them matches. Your club level matters, obviously. Its easier to give matches to a 17 year old if you play in a smaller league than if you play in La Liga, Prem, etc. At a lower level, a talented 16 year old can be a key player for you side. At that age, there is a risk of overplaying them but I haven't had it backfire on me yet.
Loaning can certainly work, but there are a lot of factors to consider. You want them to be at a competitive level for this current development, playing regularly, and preferably at a club with good facilities. If you have a reasonably talented 18 year old loaned to a smaller Prem club were they good enough go get regular matches, there isn't much point. Loaning means losing control of tutoring and PPMs, so I personally tend to keep them at the club until they hit a point where they absolutely need matches and I simply can't accommodate that.
No reason to keep them on average training unless they are very injury prone. Keep them on heavy and just watch their fitness and the medical center.
