Regarding the discussion about age: Life expectancy in the Middle Ages wasn't that drastically lower compared to today - if you exempt infant deaths. Child mortality was drastically higher back then, meaning most early deaths happened in the first couple of months and years of life. Once you managed to survive childhood, chances of getting "regular old" (70) were fairly solid. It's just the overall statistics obviously get skewed a lot if a lot more people die at the very beginning of life.
Sorry to keep that discussion going, but I do slightly disagree on that one point (everything else you said 100% agreed).
I do find 70 to be a little too optimistic, given that as you said "life expectancy" is the average above all people born. Thus that term (again, as you said) includes chidbirth mortality, which accounts for a significant amount of deaths for both mother and child. Maybe "longevity" would be a better word to represent what is the subject of debate; but then again, that word is not clearly defined.
How long a person could - statistically - expect to live after he or she reached a certain age, e.g. maybe maturity, is the thing that is for debate. However, you can't precisely talk about numbers unless you draw an arbitrary line. It all depends on how old the person is at the time you are asking - unless you take the generalized "life expectancy", which is, of course really low in the middle ages due to childbirth.
If you take a quick glance at Wikipedia (best source ever

), the problem is shown quite handsomely: First by the lack of precise numbers (not many statisticians back then), and second by the dynamic nature of anything but the (not helpful) "life expectancy".
It is said there (
https://en.wikipedia.org/wiki/Life_expectancy), under the point "Life expectancy at older age", that in Classical Rome: "If a child survived to age 10, life expectancy was an additional 37.5 years, a total of 47.5 years."; whereas in Late Medieval England: "At age 21, life expectancy was an additional 43 years (total age 64)."
Those are differnt statistical numbers, and they can't be compared properly. However, they show two things: Infant mortality matters very much, and the older you are, they longer you can expect to live.
The idal strategy would thus be just to grow older, because then, statistically, you will live longer!
Obviousely that would be a false belief, resulting from a wrong interpretation of statistics. The truth is, that you will die eventually, and the more people the same age as you die while you still live, the better off you are statistically. Those death before a certain point of age will thenforth be no longer included, compensating diminishing factors auch as childbirth, while also distortioning the picture: Those that died from the pneunomia at 9 would maybe have died at age 39 just as well. And the Bubonic Plague hardly cared about age when it struck.
Another interesting (and I think, fitting) number is the Medieval Islamic Caliphate: Average lifespan of scholars was 59–84.3 years in the Middle East, and 69–75 in Islamic Spain.
These are numbers in the category you were thinking I guess. Mind you, however, that those were
scholars. They were likely higly educated, had good healthcare (the Caliphate was much more progressive than Europe in its Golden Age), did no hard labour and weren't involved in fighting.
Not the average medieval person then
The average medieval person of course could very well grow old. But as I pointed out three paragraphs above, you would only have good chances to reach 70 if you were quite old already, let's asume maybe 25. So the game should thus be far more punishing with young rulers, and far less with olders, right? Not entirely right.
If you take a constant tick of 1% of population deaths per year of age, your additional life expectancy would stay constant (unless I did the math wrong). So you would always, let's say, may hope life an addition 20 years. In the end, this would mean that someone would still life at the end of all times, and so this model can't be useful. Additional life expectancy
has to go down with age, to get the maximum life span capped at a certain age (122 years 164 days in humans until now). So older people have to die eventually, and proportionally more of them have to die each additional year of age. In the end, 100% of the remaining ones have to die.
Sorry for this unstructured babbling about statistics, and I think I haven't quite yet come to the point (there being any), but it has gotten late, and this wasn't really a discussion about longevity in the first place was it? Sorry for derailing
Anyway, 70 is very optimistic for a medieval person to achieve. Not only child mortality, but also famines, fighting and diseases were harsh. I'd assume around 50 years for the guy who survived infanty in-game, that is at age 16, might be a good guess. Yes of course you can get 70. That's why many players don't let their ruler ride into war. Mostly impossible in reality.
Edit: Some more on this:
"However, by the time the 13th-Century boy had reached 20 he could hope to live to 45, and if he made it to 30 he had a good chance of making it into his fifties."
(from
http://news.bbc.co.uk/2/hi/health/241864.stm)