Sexually mature, yes.Actually cockerels tend to mature a month or 2 before the pullets do, I put first 2 days eggs from 2 hens that were just starting to lay 3 were infertile but the 1 did hatch a chick that was tiny due to egg size but once outta the egg she caught up quite quickly to the others from the older hens.
Personality mature, no.
There is more involved with a cohesive integration of new flock members.