My daughter is also in a L9 group with all of them being the same age. (12/13) 11 girls in total. I have found it so far to be a good thing. They push each other, support each other, and also feel bad for each other. I think it has brought out the best in my daughter. It has also taught her how to be gracious when things don't go her way and to be humble when things do go her way.
She has only been at this gym for a very short time so my experience is limited. (she came from a gym where healthy competition did not exist, it was in fact the exact opposite)
A little story. My daughter came to this gym under not so great circumstances, and it was all very sudden. She only competed 1 meet with them before going to states. The entire situation was beyond stressful for her. She went to States, 8 girls from this new team in her age group and she won the AA. A few weeks after States one of the mother's from her team called me because she wanted to let me know how very proud HER daughter was of my daughter. She knew how hard it was for her and she was just so excited that she was able to go out and have such a great meet.
That conversation showed me two things. (1) How much these girls really care about one another, even though they compete against each other, they are truely happy when one of their teammates has success.
(2) She is in a very healthy gym environment. The coaches help instill that sense of competition but also the bond that these girls have for one another.
I really believe that in the right situation, competiton amongst peers is a wonderful thing and it teaches kids life lessons that they will carry with them, and be able to rely on forever.