I'm wondering when is the best time to make the switch from Chick Starter to Layer feed for my birds. My birds are 7 weeks apart in age. The older 3 are all EEs, if it makes any difference. The brand I'm feeding suggests making the change at 16 weeks. Should I start mixing in Layer feed when the older birds hit 16 weeks or should I wait a couple of weeks until the younger ones catch up a little? Thank you!