what is the japanese word for an older, but still sexually attractive woman?

The Japanese don't have a word like MILF in their language. MILF is an acronym for English as well as relatively new slang, so it has not transfered over across the tongues.

There is a word, ハイミス [hai misu], which refers to older, unmarried woman who is often thought of as a business woman because, in Japan, woman who lead busy lives are probably unmarried. When a woman gets married she is supposed to place any career second above the house and family. I don't think it's a very common word to be used anymore, though.

Japanese Milf Thumbs

Closest to your answers are

Suteki na hito - an attractive person

Suteki na jyosei - an attractive lady (woman)

Kirei na jyosei - a beautiful lady (woman)

Utsukushii jyosei - a beautiful lady (woman) - more in writing word instead of speaking word

sonnie momma wanna kachobie

if she is married or widow woman

silver fox

Kris harvey...

Oh you said older and woman.

Hmmm...

bihaku

do you have a thing for "mothers"?

NMILF