It's also a generally accepted fact that people are taller now than they used to be. Some even go so far as to blame the hormones in milk for today's much taller human race.
Both the height myth and the age myth are false. Sort of.
See, people were, on average, shorter and less healthy a couple hundred years ago. They didn't get the same nutrition we do now, and they certainly didn't get the sort of rest and access to laziness that we do now. That doesn't mean, however, that human beings didn't have the capactity to be 6-foot-whatever and live to be 90 years old. It just depended on what sort of lifestyle you lived.
For example, a nobelman who ate well versus a serf who ate drippy horse slop would be much taller. Hard-working farmers were, on average, 4 to 6 inches shorter than people in the upper classes. And popes, who through the course of history, have lived to be well over 70, 80, and sometimes even 90 years old because the life of a pope (moreso then than now) really wasn't all that stressful.
And while the "average" age of lower class workers was right around 30 years old back then, the numbers are a bit skewed. Infant mortality was horrible back then, and if you average in all those one-month-olds with the people who died when they were 60, well... you do the math. Plus women died more often during childbirth then because they weren't allowed six weeks recovery time. So they died right around 30, too. It's amazing how we can make statistics do whatever we want them to, isn't it?
So the bottom line is this: there hasn't been some fundamental change over the last few hundred years that's suddenly given human beings the ability to grow taller and live longer. We're just eating better, working less strenuously, and saving the lives of more babies and mothers.