I think you may have misinterprered that statistic. That 30 year figure is a measure of overall mortality and not adult lifespan.

It's worth noting that childhood mortality was extremely high prior to the 19th century with some estimates putting the chances of surviving into one's teenage years as low as 50%. What that means is that, when taking the average, you're including people who never made it to adulthood. Once someone made it to adulthood, it was not uncommon for them to live comparably as long a life as many live today.

You might find this explanation useful: http://blog.cnbeyer.com/history/how-long-did-they-have-life-expectancy-in-history/

As /u/BlindPelican already mentioned, historical life expectancy values are usually skewed by high infant mortality. If you factor that out, you come to very different results.

In fact, Leonardos life span it seems was only slightly above the average age at death for Italian renaissance artists from Central and Northern Italy, which is apparently 64.05 years - I found this article in the Lancet that goes into a bit of detail on the age at death of Italian artists which I found so delightful that I had to comment here to share it with y'all: https://www.ucl.ac.uk/medical-education/reprints/1975-Lancet-LifeExpectationOfItalianReneissanceArtists.pdf

The takeaway of this is not that Italian renaissance artists were much healthier or lived longer than the general population (many of them would have more amenities and better food available than the average peasant, though, and it seems that successful artists tended to live longer - or they could be more successful because they lived longer), but that to become an Italian renaissance artist, you would have to survive infancy, so this dataset nicely factors out high infant mortality.

This is true for other periods of history as well, studying ages at death of people from antiquity you will also find higher life expectancies after surviving infancy and the corresponding ages at death (though recordkeeping is a factor of uncertainty here). Another bottleneck is childbirth, especially in young women, which also skews the numbers a bit, you will find a lot of dead women in their late teens and early 20s, but after that, their risk of death decreases until they reach old age again.