Well, that deopends on what they mean by "action":)...
If they want to be near the most shopping and dining, they need to be near Pier Park. Calypso is closest (across the street), and Aqua, Sterling Breeze, and Tidewater are also relatively close. I'm not sure about minimum rental age at these places...some say 25+ now, so you'd need to check.
If "action" means walkable to the clubs like LaVela & Spinnaker, they'll want to be on Thomas Drive. The condos immediately surrounding the clubs are older and I would recommend renting on VRBO so you can see photos of the actual units you are getting (if possible). The old stand-by is The Summit, and on the other side is Moondrifter. Sunbird is also relatively close and I believe all of these places would accept 22-year-olds, but again you'd need to check. Keep in mind that as these condos are older, they can be very nice or very dumpy depending on the owners...hence, the reason I recommnend VRBO instead of the on-site rental agencies.
In the buildings I mentioned near Pier Park, the units are newer and unit upkeep is less of a critical issue.