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I think many of the same things apply across ages. Pay a reasonable wage. Allow adequate time off work. Show a path of steady advancement. Strive for a pleasant work environment. Even with those "needs" met, I would still expect there to be somewhat higher turnover with younger staff. To some degree, that is just part of being young (and sadly I'm old enough to consider 20s young). Trying new things is part of life, and for most of us the 20s is the last time when one can do pretty much what one damn well pleases. It is much harder to make career changes and take risks when you also have to think of your family. And once a child comes along, those roots start sinking deep.
I saw an article in CIO that talks about this issue. You might find it worth checking out: