In some cases, older chickens can kill the new ones even if they are the same size, which is why you have to be so careful. Although 16-18 weeks is a standard for me, it does vary on the chicken. Some people have had success introducing 8 week old birds with mature hens, or even younger chicks!

What Age Can chickens eat pullets?

WHEN CAN I INTRODUCE THE NEW CHICKS TO THE FLOCK? Baby chicks must be raised on their own to an absolute minimum age of six weeks old before being introduced to the rest of your flock. If possible, wait until your pullets (young, non-laying hens) are 8-12 weeks old before making the introduction.

Will a hen kill a baby chick?

A first-time mother hen is in danger of injuring or killing her babies, especially if her first-hatched is the first chick she's ever seen. Whether the chicks hatch from her own fertilized eggs or from fertilized eggs you've purchased to hatch under your broodiest hen, the risk is the same.