I am not an old timer but I do agree with some of the others, if they love it, you can't keep them out of there. The Fellows would live at the gym if I let her, seriously.
She started preteam (level2) at 5 for 4 hours a week, the level threes do 6 hours a week but we went straight to four at age 6 and jumped to 9 hours. She was thrilled and it really didn't effect her. I was worried, but she didn't miss a beat. She just turned 8 and will start 12 hours a week in a few weeks training level 5. She has been asking for another day at the gym for several months now, so again she is thrilled.
Only injury was a dislocated elbow in a freak backwalkover accident (she is hypermobile in her elbows). Other than that nothing but some sore muscles on heavy conditioning days.
I guess she may decide some day she doesn't want to do gymnastics anymore and we will support whatever decision she makes, but she has big goals and as of now sees herself staying with it. I have never once had her say she didn't want to go to practice. She has always been the one driving.
Homework is a challenge, but she is learning how to be efficient as all gymnasts somehow miraculously do. This has probably been the hardest transition, but she is in a challenging academic program. All the girls at our gym are very good students. We get it done and it is becoming easier even though the homework is increasing.
She has playdates and goes to parties, takes swim lessons and goes to camps in the summer just like all the other kids. She may not do as many of these things as other kids but definitely doesn't miss out on much.
Getting enough sleep, water and eating well is absolutely essential. We have emphasized this with our kids since they were babies and I think it absolutely contributes to her ease of handling all this. She can definitely tell when she doesn't eat right before practice or doesn't get enough sleep for a few days in a row. We have a no hot lunch on gym days rule. She really likes to get hot lunch with her friends, but the food is crap so only on non-gym days and only sometimes.
She understands that she must take care of her body and mind and be more responsible about things than the other kids her age. We talk about these things and teach her about how to make good choices for her body. As a result, we are all making better choices for our bodies too

She loves gymnastics and will happily do whatever it takes to make sure she can be at her best.