Can't speak for the OP, but my wife and I consider ourselves "mature" at early and late 50's, respectively, and wouldn't take offense at being referred to as such. We also agree that by our 40's we could have been considered as mature, 20's and 30's, young. I suppose that in "pornspeak", you see people in their 40's in the mature catagory, even as young as 30's, which surprises and depresses me, ha. An age fetishist, well, that would be up to the individual's perspective. Have to say though, we don't think in "pornspeak" or "fetishspeak", so YMMV.
Agree with Diane and others who say that sexual openess has come with maturity for us. Much more shy to express our desires when we were young, far more comfortable with our sexuality at this age.
I think some here demured from identifying what they considered to be "mature" because they thought the phrase was offensive to some, but I didn't hear that from anyone, seemed people just wanted an age range for the sake of discussion.
And.. I just looked up and saw the date of the last post. Oh well, I already typed this sucker. ๐ Maybe this is still a subject of interest..