If you demote them after ST starts, you make take a demotion hit.
If you leave them in the bigs and play them, they'll develop just fine. However, you will be moving them closer to arb and FA.
Age isn't important(to me). If they aren't 3-4 year pros, they could probably stand some more time in the minors. If they have 3 pro years, they wouldn't have to be on the 40 but they could still be ML-ready.