Earlier quoted context omitted.
It was pretty rare even among medieval kings to live to be 70. The first English king to be definitely alive on their 70th birthday (though no longer "in office") was Philip of Spain (jure uxoris) in 1597, so not a medieval king. That is Early Modern Age. Elizabeth I. didn't make it, though barely, and so the next to reach 70 was George II. in November 1753! Only since the second half of the 18th century is it common…
Medieval kings were warriors and very often victims of assassination, so they had a way lower life expectancy than a typical peasant of their times.
Assassination definitely brings down the average. But a fair number of English monarchs managed to die in bed. (I was gonna write British, but no: the Scottish kings practically never died in bed. Unless they were stabbed in their sleep.)