The best age is 4-5 months old before putting juveniles in with adults. And before that they have to be separated by a fence where they see each other but don't co-mingle for about a month with each other before removing the fence barrier. Even then there will be a pecking order they'll establish amongst themselves possibly with some pecking or squabbling. We allowed our 4 month old juvies time on one side of a fence to let the older hens get used to the newbies. Then at 5 months they were put together on the main flock side. It's less stressful for all birds involved not to have to fight so much for territory because they are somewhat acclimated with each other. Never put smaller/younger birds with older/bigger birds. At 5 months the Silkies are big enough to be on equal size status with the adult Silkies. Just MHO.