As anyone who has ever logged on to Nextdoor.com knows, Philadelphia is crawling with millennials. It turns out Pennsylvania, known for an aging population, is home to quite a few of the nation's most millennial-dense zip codes.

A new study by RENTCafe ranked the zip codes nationwide for what they called "millennial share increases." In other words, they measured the increase in population in terms of share and overall numbers of renters in the millennial age group.

Of the 20 zip codes in the country with the greatest share increases, two were in Kensington: 19125 and 19123. In 19125, the neighborhood demonstrated the eighth-highest rate of increase, with 37.4 percent growth for millennials — a raw number of 11,200. The 19123 zip (20th place nationally) showed 33.1 percent growth, which translates to 7,300 millennials.

In terms of largest share outright, two Philadelphia neighborhoods claimed top 20 spots. Manayunk, in 19127, was the second-most dense zip for millennials (at 71 percent). Center City West, in 19102, took the 19th spot, with 60 percent millennials.

Check out the whole report, which also offers a look at zip codes by state. Pay special attention to the zips in Pittsburgh for a look at the youngest up-and-coming zips in Pennsylvania.