Are you going to have a vehicle? If so, go to Blu Martini, and Cadillac Ranch at Town Square which is located just south of the strip past Mandalay Bay and the "Welcome to Las Vegas" sign. All of the places I have been to, are normally 21-26y/o, out to get plastered, sloppy, and act like they are Brody Jenner or Paris Hilton.
Voodoo Lounge at the Rio has a club on the 51st floor of the hotel and the outdoor patio overlooks the entire Las Vegas Strip. You can catch the free shuttle from either Bally's or Harrah's but it only runs until 1am...take it one way and get a cab back to the strip. Great mix of people...you may see some salt and pepper folk if you go too early.
I'm not sure about Tao at the Venetian, my friend went there and had a blast...she's 39y/o, but I'm not sure of what she was dancing with! lol
Stay away from Pure, LAX, and Jet...probably the worst for under 25y/o crowd.
I'm not really a "club" person as I hate being bumped into all night by mascara running clutzes, but these little pieces of information is what I've heard from others. I've only been to Voodoo, Jet, and Foundation Room at Mandalay which we had a members pass to get in with. I think one night during the week they do charge admission to get in, but no idea which day and how much.
Check out
www.lvol.com, click on Las Vegas Nightclubs for descriptions of each since I'm not a whole lot of help...
Check out
www.vegas