Girls tend to mature faster, so I found older girls to be a good influence. Of course it depends who it is, but the older girls we knew were more like mentors than running the risk of exposing the younger girls to older subjects.
As far as our son goes, I like him to be the oldest. He does well as a leader and is very kind to younger boys. Younger boys adore him and he is a good example to them. He follows his dad all day at home; it is good for him to practice leading.
Nope, I can’t find anything in the Bible to support this practice. Other than it could be an early practice of the younger women being taught by the older women and for our son a chance to practice leading so he can later lead his family well.