20:1 specifically?

https://www.ssa.gov/oact/STATS/table4c6.html

That's just the odds of Biden (or Trump for that matter) keeling over for literally any reason whatsoever based on the social security actuarial table. So if I can get 20:1, I should be break even betting against any octogenarian.

There is a little bit better than a 1 in 15 chance, that for any 81 year old, they'll die that year. So I hedged it to 1:20 as insurance, because I figured I'm really only betting on the first 9 months of the year. If I can get 1:20, that's break even odds (actually slightly in my favor). Also, figure the presidency, campaigning; that shit aint a walk in the park. Probably more likely still to die in office than a standard octogenarian.

I'll give you 10:1 if you bet at least 50 bucks. I lose, you make $50; I win, I make $500. Hows that sound?

Edit: As to how to enforce it, we can make a community and pin it there. ITs similar to what we used to do in a bar I frequented where we would bet pints (very similarly) on whatever was happening. There was a cork-board and bets would get pinned to it. And I trust you.