Well I think what you said is OK. It's probably just a regional thing where 'mature' is sometimes a polite way to say older. However let me ask you this, when you hear immature, do you think aged 39 and below? Personally, I think mature and has less to do about age, and more to do about about adult themes and content. Like mature = "not for children". I am also extremely glad they're not under 18.

MATURE YURI IM SEATED!!