I am starting to conclude that "playing up" just does not work (in general). I am talking about playing-up from an A level younger team to a same A level older team. (don't want to even consider going from a younger A to an older B..... I am not talking about those cases.)
I think that even if the younger girl can compete in the older squad, she will still get mental "wounds" from the older players and environment. I think for it to work the player has to be a top 10% player when they get to that older team. (to automatically have the older girls respect)
Keep the girls in their own age brackets.
my .02 ......
I think every girl is different. You need to have the maturity to handle playing up. As far as respect, it is no different than any other team you would join. You have to earn it. For my DD, there has been zero issues playing up. I see her bringing her max intensity, every time she steps on the field with older girls. She is constantly pushing herself to better performance.