I did some research on lifespan that I posted here some years ago but can't find it again but a rough summary is that if you managed to live to the age of 21 (and weren't dirt poor!) you'd stand a very good chance of living into your 70s and 80s, it was childhood deaths that really brought down the average lifespan in past days. So looking at the average (which usually means the mean) lifespan isn't that useful if you want to argue that we don't see many cancers because people didn't live long enough in the past to develop cancers.
Obviously many, many more people do now live to old age when compared with past times now that childhood is not so hazardous (in developed countries) so I agree that describing cancer as a "disease of old age" is a good rule of thumb.
ETA: Found a link to show what I was meaning:L
http://apps.business.ualberta.ca/rfield/lifeexpectancy.htm
That's a common myth, actually, but for Ancient Egypt it's clearly and provably a myth. The Egyptians were in the habit of writing down stuff. Including stuff like, you know, when someone was born and when they died. They wrote that stuff when burying someone. And we have a butt-load of such data points from them.
In fact so many that we can plot a curve. X axis being how old that guy was, and Y axis how many dead people we've got of that age.
And it basically looks like a two-hump camel. There's a big mortality spike in the first 3 years of life, and after that it looks pretty much like your average Gauss curve.
And the peak of that one, in the Old Kingdom times is around 35 years old for men. That's it. That was the life expectancy you actually had if you got past the infant mortality fluke. Chief cause of death were actually tooth abscesses, as they had sand everywhere, including in bread, and it literally filed teeth down to the nerve over time. And here's the scarier part: for women, who had that AND childbirth to deal with, that peak of the death curve is around 25 years old.
By New Kingdom times, the life expectancy if you survived infancy had increased by about 10 years for both genders, so now it was mid-40's for the males and mid-30's for the women.
Of course, as usual this varied a bit with social stratum too. For the rich they lived longer, the poor shorter. Upper class women especially had much better chances as they tended to get married off later, which greatly reduced the chance to meet Osiris in the first childbirth. (The poorer were married off at the age of 12.)
But even the rich still got their teeth eventually filed down by sand. Some of the teeth on those mummies look like they were cut in half with a saw. So even there life spans anywhere near modern ones were rare outliers, not the norm.
I suppose what sometimes confuses people is the fact that for Egyptians the number 110 was of special importance because of numerology, and defined as the perfect life-span of a human. So some people expect that they must have at some people getting close to that. But really it was just based on numerology. There are no actual data points that come even close to that. They also had the confusing habit to occasionally say about someone or another that he lived 110 years, but that was just an euphemism for "he lived a perfect life" (110 being perfect) or in other words "he was a really nice fellow and liked by everyone". But if you look at those birth and death years on his plaque, it turns out they were burying him at 35 years old. So, as I always, say, he must have gotten bored to death in that tomb for 75 years if he actually lived to 110
