I take the mature woman thing to mean more along the lines of a non-virginal type of thing. Like for a young, virginal girl it might be more difficult to use? Idk...
Probably more along the lines of they don't want virgins to use it because people might get in an uproar-- "She'll rupture her maidenhead!" "It'll lead to masturbation!"-- that sort of thing. Ye Olden Dayes stupidity.
I'm pretty sure that's what it means, too, and I think "mature" woman was code for non-virgin. Sort of like R-rated movies are "for mature audiences only."