I've always had a hard time with this also. I agree a milf has to be sexy to make it in that category. I think she should be 30 and under, mature 30+ and granny 50+. Most of the time I can't tell how old she is by looking at her unless she is wrinkled
I'm in my forties, not wrinkled, a grandmother but I don't think I belong in the granny category yet
Lately I've been sticking with the really wrinkled granny age which is much easier.