Hi Amelia.
I'm around your age too and face a similar dilemma. Personally I think that all depends on your looks and how your body/face looks. I know 38 year olds who don't have a wrinkle on their face with a super fit great work-out body who don't mention the word "mature" (but they do mention their age) on their ads and they're busy as hell. I would say if you have a "mature body" (a few wrinkles, saggy breasts, a stretch mark or two, some cellulite etc... Meaning: You have a "normal" body for a woman your age) then I would market myself as "mature", but if you have a bikini body, no wrinkles whatsoever and are one of those lucky ladies with great genes who can pass for a 26 year old I wouldn't market myself as mature. Not yet.