Adapting a suggestion that I've heard and haven't studied too closely: Find insurance companies that essentially bet that people won't die in the next N years, find very large companies of this sort or form some kind of aggregation of them, and make the case to them that if they spend a bunch of money funding the development of some treatments that work, then they will make a profit because their bets will turn out better.

These companies exist, they sell a product called life insurance...

Yes, I was referring to that. I'm just not aware of such companies doing large-scale funding of life extension research to increase profits. I suspect it could be done, just that you might need to aggregate a lot of insurance companies to make it profitable.

Ah, but the other half of life insurance companies frequently sell annuities, which are bets you'll die soon enough. :)