I don't think that older kids are more or less likely to catch her. I think that it's a smaller person, a larger height, and a 9 year old is gonna have trouble bonding with a 15 year old. She can look up to her, she can ask for advice, but it's hard to keep conversation going with someone half a generation younger.
Also I doubt that gyms would have youths flying, juniors basing. I think the smart ones would have 2 junior teams (if that's allowed in the US, I wouldn't know.) One with youth, one with junior so that when the juniors turned 16, they will have great flyers and bases for senior. In short, the teams won't change, just the division.
Thanks for disagreeing, can we agree to disagree?<3