That's a myth. People in Mideval times generally live about as long as people do now provided they actually made it to adulthood in the first place. Once you hit you're teenage years there wasn't really stopping you from making to 70 or 80 other than general unforeseen circumstances like war or natural disasters. Really it's more that child mortality rates were so high back then compared to now and most kids wouldn't live past their 5th birthday.
It's no myth. There were actually people who lived to be over 80 back then. But they were the big exception. Yes, there was a very high child mortality rate. There were many wars, environmental disasters, epidemics and droughts. And they just killed the people. In addition, medicine practically didn't exist or was in its infancy, not to mention hygiene back then. Last but not least: there were only a few people. So they were more or less on their own. There were no retirement provisions or a welfare state. Anyone who wasn't fit and/or had a family to look after them died. That was the case.
If you're ever interested in medieval cemeteries, you'll notice that the majority of people didn't live to be very old. If they had had the knowledge and options we have today, things would definitely have looked different. The circumstances of the time rarely allowed for old age