My daughter experienced a similar situation - for a while she was contemplating quitting or changing gyms, because everyone in her training group was always 2-4 years older than her. They were all really patient and sweet with my daughter - she could be a handful at times when she was younger, but she didn't understand why she wasn't always included with the big girls. They were starting to get crushes, get their periods, and in their tween years. My daughter was still into unicorns and slime, lol.
I had to sit her down and explain to her, that they were in middle school, and she was still in elementary - they loved her and some of them even babysat her and my youngest, but because of the age difference, sometimes big girls needed their time with girls in their age bracket.
Even though they had sleepovers, and shopping dates without my daughter, one thing that helped my daughter feel more included was when we started doing post-meet hangouts. We would all go out for starbucks or ice cream after a meet, that way she felt included in hanging with the big girls outside of the gym.
Now she is still the youngest on her team, but because she's in middle school, and the age gap is only 1-2 years, she has a strong bond with her teammates. They have sleepovers, and shopping dates, and a text message thread.
If your daughter is one of the younger ones, things will likely even out a bit as they get older, and in the meantime, it might help to have some sort of ritual or group date outside of the gym that includes everyone as a team bonding type of gathering. Taking the lead on that as a parent is something we did when the girls were younger, but now that my daughter is in middle school they all handle that themselves. I still check in with her to make sure that no one is feeling left out!