I'd actually wonder a bit about the fit of Princess for 20 somethings. Seems to me, that onmajority of my experiences n Princess they attract a bit of an older crowd. And Princess draws alot of repeaters.
It really depends where and when you go and for how long. There's no guarantee as to the mix of passengers on any particular sailing, of course, but seven-night cruises to Alaska, the Caribbean, and the Mexican Riviera during school vacation periods tend to draw a much younger crowd than longer cruises, cruises to other destinations, and cruises at other times of year, but you'll find a lot of young adults even on some cruises that violate these criteria. A few years ago, I was astounded to find two hundred (200) children aboard the original MV Royal Princess (45,000 tons, 1200 passengers) for a twelve-day cruise to "Western Europe & Scandanavia" in early June. Nearly all of those children brought their parents with them, so there were quite a few number of young adults aboard. Alas, school vacations generally do not begin until late June here in New England so I did not realize that they begin around Memorial Day in other parts of the country.
Most passengers also tend to interact with a lot of members of the crew who are "twenty-something" or even younger during the course of a typical cruise. If one is on a cruise where there are few young adults among the passengers so "night life" is pretty dead, one usually can party with the younger members of the crew in the disco.