Quite a few gravure idols now are starting in their mid 20s now, I think there might actually be more of a market for it (but that's just an assumption based on the popular gravure idols I see crop up a lot). Ino Enohara started at 26, Minami Hori and Aoi Mizobata started at 24, etc.
Gravure idol scene I think has changed a lot in the last decade - the whole idol world has, way, way less money in all of it than it used to (gravure especially because of the death of print media). I mean, you're still getting 12 year olds debuting in Hello! Project but they're like a dinosaur that has just pared back its business model as much as possible to survive.