The only real "kid" gymleaders I can think of from Hoenn are Flannery and the twins. Everyone else was a legitimate adult, albeit some were younger than others. Norman, Winona, Juan, Roxanne, and Brawly are all "older" trainers. The only one who you could argue to exclude from that group is Brawly, but I always saw him as in his mid-20's. Winona and Roxanne are both older women, probably a little under 30 is my guess. I don't see how you saw them all as "kids".
However, I will agree that it would be great to have more older gym leaders. However, I don't want a majority of them being older. I think a good split would work out well. While the older trainers do have more experience, a younger trainer could just be a better trainer (I mean, you are a child yourself who is beating these people). So I think it's unfair to see the younger gym leaders as inferior just because they lack experience. It's all about skill level, regardless of age.
So would I like to see more older characters? Sure. Do I think there should be a majority of them? Nope. So I really don't care how the ratio works out, I just don't want every gym leader I face being some older dude with a serious mustache.