Why haven't the good people at GiveWell written more about anti-aging research?

According to GiveWell, the AMF can save a life for $3.4e3. Let's say it's a young life with 5e1 years to live. A year is 3.1e7 seconds, so saving a life gives humanity 1.5e9 seconds, or about 5e5 sec/$.

Suppose you could invest $1e6 in medical research to buy a 50-second increase in global life expectancy. Approximating global population as 1e10, this buys humanity 5e11 seconds, or about the same value of 5e5 sec/$.

Buying a 50-second increase in life expectancy for a megabuck seems very doable. In practice, any particular medical innovation wouldn't give 50 seconds to everyone, but instead would give a larger chunk of time (say, a week) to a smaller number of people suffering from a specific condition. But the math could work out the same.

Of course, it could turn out that the cost of extending humanity's aggregate lifespan with medical research is much more than $5e5/sec. But it could also turn out to be much cheaper than that.

ETA: GiveWell has in fact done a lot of research on this theme, thanks to ChristianKl for pointing this out below.