Whenever I think of my ancestors, I often wonder what "old" meant. When I was a kid, "old" meant anyone over thirty. My brothers and I have a joke among ourselves that our grandmother was "always old" - we remember her from her fifties to her nineties. She lived to age 99. That's her at left at age 16, so she wasn't always old!
So, what is a "normal" lifespan? Well, the cold hard fact is that if your ancestors lived long enough to have children, that was enough. If they lived long enough to have children and raise them to an age where their children could have children, that was perfect. So, in genealogical terms, forty is as long as anyone really needed to live. And that is being generous! Do the math.
But if you look at your ancestors, you will find that averages and medians and such don't really mean anything. Lifespan is as individual as fingerprints. And genetics really tells you nothing. One ancestor may have died young, another lived to a ripe old age. As I write this, my parents, born in the 1920s, are still going strong in their eighties. And it has nothing to do with modern technology - we have several ancestors who lived way beyond their forties.
So, if you are wondering what a normal lifespan is, even for many, many generations back, there really is no answer. A rule of thumb is a generation is twenty years, but some people have children much older than that, and some younger.
Life finds a way.