A lot of women are no longer sexually active at 80 or in old age but it still is possible for a woman to be sexually active at age 80 and above if they're interested in the sexual activities.
Some women stop being sexually active around 55 to 60 years old but some women may be sexually active longer and well into older age.
And if the 80 year old women is sexually active they should still practice safe sex and the guy should use protection because although rare it's still possible to get pregnant at age 80 and if you do not want to become pregnant at age 80 or above then use protection.