I don't think that it's age so much as size.  You really want to wait until everyone is about the same size, though if you're referring to a silkie roo being integrated in a LF flock, you may be in for trouble.  Due to general fluffiness and fluff-obstructed vision, silkies tend to present "pick on me targets" for other chickens, particularly LF with unobstructed sight.  So he might be in for a rough time, and I'm not sure how he'd do fertilizing one of those big fluffy-rumped hens.
 
If I were you, I'd probably wait until your fellow was crowing and "raring to go" and then put him in a decent-sized crate (with food/water) IN the run with the rest of the flock for a good week or so.  Then start free-ranging him with them after 3-4 days.  Let everyone see everyone else and soften the blow to the pecking order once you loose him on them.