Thats not true. Mediaeval people live relativly long! Expecial the nobility. The short living story is a lie from the Age of Enlightenment. Ages of 60 are really common.
You're only partially right. Life Expectancy in Medieval Britain from birth was only 30. However, if you managed to survive to 21 then you could reasonably expect to live to your sixties <wikipedia>. Up until modern times kids died a lot before the age of five (one of the prime reasons for having large families, at least one will live), and young mothers, especially, died in childbirth or due to complications with carrying.