(MoneyWatch)  Visit Boston and one of the first things you're sure to hear is that, for a big city, the population is among the youngest in the country. Jam-packed with universities, such as Harvard, MIT, Boston College, Boston University, Northeastern and Tufts, a whopping 72 percent of the population is under the age of 44.
But when it comes to staying young, Boston ranks No. 7 -- well behind San Francisco, where the average age has shot up to 38.5, according to RealAge.com, a website that seeks to gauge your age based on a different yardstick.