a 320 year old elf marries an 80 year old human: Is the elf robbing the cradle, or the grave?

Seems to be the elf is setting themselves up to receive the human's inheritance money. If they play their cards right they could pull this scam many times

Ah, the practical problem solver.

You need to be specific about which type of elves this is.

If we're talking LotR, elves are immortal.
If we're talking 40k, Eldrad is 10,000+ years old.
If we're talking WoW, it's a couple thousand years.
If we're talking D&D, elves live like 750 years.
If we're talking Shadowrun, Elves lit's all over the place.

Keebler
- Also immortal. And hypersexual.
- Damn it, I knew I should've invested years of my life into cookie lore for a brand not sold in my country!
  Probably more useful than 40k lore, but less useful than LotR lore.
Isn’t Mrs. Claus technically an elf?
- Santa himself has been described as a "jolly old elf."
  
  I'm no lore expert, but it makes me wonder if the original Santa (human) passed the title to one of his former underlings. The Dread Pirate Roberts of the North Pole, perhaps.

Is 320 considered an adult for elf aging in this scenario?

It depends on what state you're in.

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.

There's no way an 80 year old woman has the life experience to compare to a 300-1,000 year old elf. There's an inherent power discrepancy.
- I mean yes but it's diminishing returns. The rule kind of falls apart at these numbers.

Do elves and humans intermarry? Would that be like beastiality?

I don't think their DNA has been sequenced, but I'm willing to bet someone made babies with Homo floresiensis. I think bestiality must be a no-babies thing. As far as I'm concerned Homo floresiensis is blurry memory elves. Maybe weak, but I plug my no vote.
That's how you get Bretons
Although often considered a slur nowadays, Half-elves were the most common interspecies offspring in D&D.

It would only be considered beastiality by racists that view elves (or more likely, the humans) as animals.
- Where the heck is half-elf being used as a slur? This is the first I've heard of it.
Yes, first quoting Sir Terry Pratchett:

J.R.R. Tolkien has become a sort of mountain, appearing in all subsequent fantasy in the way that Mt. Fuji appears so often in Japanese prints. Sometimes it’s big and up close. Sometimes it’s a shape on the horizon. Sometimes it’s not there at all, which means that the artist either has made a deliberate decision against the mountain, which is interesting in itself, or is in fact standing on Mt. Fuji.

As such let's look at the Granddaddy of all modern Medieval Fantasy, in Tolkien's work there are several human/elf couples of renown, from Beren and Luthien which are not in the movies so people don't usually know to Aaragorn and Arwen (who are a central point of the movies). Not only that, but we know that they can produce offspring, because Elrond is a half-elf, not only is he a half-elf, but both his parents were half-elves.

In short, yes, they do.
- Eärendil is the GOAT of half-elves 🌠
Falka did in The Witcher, at least.

I'm more concerned about the implications of the word "robbing". I mean, I'm hoping there's consent happening here ...

/s

The way I solve such matters in fantasy, once the characters involved have reached the required maturity and properly comprehend their commitment to one another it's all good.

The grave, I would expect "From the cradle to the grave" to be describing the subject. So if the elf is marrying the old woman, then he's robbing the old woman's grave

Yes

Whatever you do don't ask ChatGPT or Gemini this, their boring ass Disney-fied answers ruin the joke.

Ahh, it's a good thing you brought AI into this. I had almost gone a full minute without reading disparaging comments about it.
- In Soviet Russia, AI goes almost a full minute without reading disparaging comments about YOU!
- Well I wasn't really disparaging AI in general, just Gemini and ChatGPT specifically, and not because they're AI, but because I feel their answers are genuinely neutered a bit aka "Disney-fied" which for this kinda joke is ehhh.

Both?

The grave