It really depends on the gym and their insurance.
Both of the gyms where I work are small, privately owned facilities. One allows youth aged 8 and up into cardio dance classes (Zumba, Fierce Funk), and weight-based classes at age 12. Another one allows full club access at age 13 as long as a parent is in the building until they’re 18.
Once you find a place that caters to your daughter’s age group, it’s still up to you as a parent to determine which classes are appropriate for her, based on her fitness level and your comfort with the instructor and the movements.