Uh, I think everyone has a different perspective on this one. Out of my experience as a coach (which is not too long) the development of a gymnast depends on many factors of which the coaches experience is an important but not the only one.
An experienced coach is a lot safer on spotting techniques even if the skill goes wrong. As we all know spotting is an important part of skill progressions so of course it has influence on how fast the gymnast progresses and wether he/she reaches her/his full potential. It is also essential to have spotting experience at lower levels. You can't just go off spotting double backs just because you did them as a gymnast.
There are not 2 gymnasts on this earth whos progressions, strengths and weaknesses are exactly the same. The longer a coach has been on the stage, the more gymmies have gone through his hands. He/She has ssen a ton of different learning types and struggelings on the way and is able to react quicker on new situations, than a young coach. Also they get a feeling for the pattern of how a certain age group lives, feels and learns.
Still, a young and unexperienced coach isn't necessarily a bad thing. If they've been gymnasts themselves they're often closer to the feeling, they bring in a lot of new ideas, motivation and enthusiasm. Those things have a very positive influence on the gymnasts development.
I -as a rather unexperienced coach- would however refuse to coach without the presence of an "oldie"
And let's not forget there are all those other factors such as the gymnasts work ethic, the gyms equipment, the time spent in the gym, the gymnasts ability to evaluate him/herself, his/her family situation, the situation in school...you know what I mean.