HK, I'm so sorry to hear that your DD is going through this rough patch.
This, too, shall pass, but I'd want to better understand why she's suddenly decided the fit is poor with her ability "peers". Was there a precipitating event that she has internalized but not voiced to you? Was there a subtle dynamic to her group that you didn't observe but which changed? Did she take a more than platonic interest in an older boy and get rebuffed? Have the older girls she socialized with begun to perceive her as competition and backed off? Is she internalizing academic stress or perceiving the next step with any anxiety?
Lots of questions, I know, but my sense from your brief exposition is that she isn't being attracted to the younger children, but is recoiling from some aspect of living as an older person. Obviously this isn't an experienced parenting opinion, but I have BTDT somewhat as an accelerated preteen and can sympathize personally with your DD. Life became complicated quickly around 13-14 when older friends' boyfriends/crushes took an interest in me. The social blowback was debilitating and required basically bottomless self-esteem to overcome it.
