Can only answer as a parent, I've seen this quite a bit at our gym and this happened to my daughter, she spent two days in the lower level group and then 1 or 2 days in the higher level group - but she was sort of between the two groups in skill level. She had definitely outgrown the lower level group, but loved being with friends her own age and like your dd being the demonstrator.
At the time when watching her train with the higher level group I could see that she wasn't up to their skill level and at times she was a bit out of her depth. The girls in this group were older, and that worried me at first, but they were lovely and treated DD as an equal. DD was so thrilled to be "training with the big girls". Interestingly the coach (same for both groups) commented that she worked much harder when training with the higher level group.
I don't think that it stopped dd from moving forward, she is now full time in the higher level group. The age range is from 8-16 years, so it is a group based on ability not age. I think it really depends on the coach, in any group of girls, even of the same level there is going to be differences in their skill levels and abilities, a good coach is able to accomodate this.