Mature

mature 

grown up, being adult.

mature 

for adults only, as in "intended for a mature audience'

mature 

for porn sites, mature refers to any woman (or man) over the age of about 40 or so.

Mature is a general catch all for pretty much anyone from a MILF to a granny. The mature market has slid down in age a bit at times because of the inclusion of the MILFs, who are often under 30 years of age. Mature women are sometimes refered to as a cougar.

In porn, the progression is generally teen, coed, babe, hot wife, MILF, mature, then granny.