What route are you sailing? I'm sure I could go to Princess' website and look it up myswlf, but, nah.
Grand Princess is a marvelous ship, my wife and I sailed on her last February for our honeymoon. We did the Western Caribbean route (Princess Cays, Grand Cayman, Costa Maya, Cozumel). I'm not sure if they've changed the stops or not.
I'm not sure how many 20-somethings you'll encounter, however. We realized that Princess seems to appeal to a somewhat older crowd than 20's or 30's. We saw only a small number of people our own age. The ship itself seems geared this way: everything on board is more elegant than exciting.
Don't get me wrong, it's not a bad thing at all. It's just not a party ship. Sure, there are plenty of bars and the food is superb, but it's not going to be a rock-and-roll-all-night (and party every day!) kind of vacation.
Think more in terms of an upscale resort, or one of the high-end hotels in Las Vegas (Mirage, Venetian, etc.). You will NOT be disappointed, however. She's a beautiful ship and the service and quality of everything is top-notch.