I've heard the term "grave robber" being used that way, although I always found it distasteful.
Urban Dictionary:
The opposite of a cougar, a woman over the age of 18 who dates older men. Differentiated from a gold digger because the men the Grave Robber dates are not necessarily rich. Also not considered jail bait, as said woman is over age, however if she was under age she would be considered jail bait.
"Ick is totally hitting on Jason right now..." "I know she's such a grave robber"