Neither. There is an age difference, sure, but an 80 year old woman is a well-lived human adult who can make her own choices. If it were an 80 year old female elf, then possible. Not sure what the age of consent is in elvish customs, or the age of emotional maturity.
As for robbing the grave, no, she’s not dead. Robbing the cradle implies young life inside the cradle and there is nothing living in a grave. Now, if the elf provided the woman with prolonged life through elvish ways, then technically, he’s robbing the grave.
Glad someone said this, it bothers me even with human ages. Like there's this perception that as you get older you simply gain knowledge, wisdom, world experience, etc. Not a lot of people account for biological limits for knowledge/memory, nor degradation from aging.
If some young intern decided to try to have sex with Biden, I think there's genuinely a conversation to be had about if that's statutory rape. I think you'd need a healthcare professional to rule on if Biden has the mental capacity to fully consent. Similar to a drunk person. They're still obviously a person able to think/engage with the world, but they're heavily impaired and unable to fully consent as a result. Age impairs cognition too.