I think that a crowd-funded prize should be created that will be released to:
The first group or individual that creates a treatment to extend healthy life by more than five years, on average.
Obviously, there's a lot of wiggle room in the definition of done here- good evidence would need to be provided to show that the treatment does what the creators claim it does. There would have to be a discussion of acceptable vs unacceptable side effects, it couldn't be something that already exists like exercise, etc.
The constraints would go on and on, but assuming they could be worked out, this seems like an ambitious but not impossible goal. Other features that I would like to see in a longevity prize are listed below.
Partially, I would like the prize to be crowd-sourced so that I can donate both a lump-sum from my savings and part of my income on a regular basis. I would guess that the sort of people who would donate to a fund like this are educated, reasonably well-paid and willing to donate larger than average amounts.
I have no data to back that up, it's just a hunch.
I also distrust the motives of large organizations who might pay for a prize like this, because they have shareholders and business interests. Their goals might not be aligned with the goal of the prize and I think it would be better to avoid corporate involvement completely.
Invested until awarded
I think that the money should be invested in a safe-ish blend of ETFs and bonds until it's awarded. The longer the prize remains un-awarded, the larger it would grow (assuming nothing catastrophic happens to the stock market). The larger the prize is, the more it encourages the development of anti-aging treatments.
Resets after being awarded
This might seem obvious, but I think that the day after the prize is awarded, it should reset and begin accepting donations again. The new goal would be almost the same as the last but would take into account the previous technique. So the next treatment must be able to:
Work in combination with the previous treatment, providing ten years of healthy life total.
Replace the previous treatment and provide an extra ten years of life on its own.
The goal here, obviously, is to accelerate the arrival of a longevity escape velocity and maintain that velocity once it's achieved. As far as I know, nothing like this exists, but if it does I would appreciate it if someone could drop me a link in the comments.