Agreed with Thefellows. There are certain points both in gymnastics progression and in age where departures seem to be concentrated. By us, on both girls' and boys' sides, a few tend to leave after a year or two of competition as compulsories. Both start at L4, so this would be L4 or L5. The next set of non-idiosyncratic departures look to me to be related to one of two things or the interaction of both: plateauing on skill acquisition as optionals and getting busy in high school around age 13-14. One interesting recent development here though is that I'm seeing a few JO kids dropping back to XCel or JD in this situation rather than leaving gym altogether. On the girls' side, it's direct transfer, but we have seen a few guys who quit come back after some time out and pick up again with JD.
My DD is an example of the latter -- made it to JO L8 but then stopped progressing on skills due to a long spate of injuries, switched to XCel just this past summer, and is now pretty happy with life in the gym. I can't say for sure, but at this point I like her chances of graduating from high school as a competitive gymnast, which to me is huge success in this sport.