I think Celebrity will be a good choice if you like somewhat of a more upscale environment and a more mixed age demographic.
A rule of thumb is that the average increases with the longer the cruise.
It really depends on what you are looking for.
Royal Caribbean is going to definitely be a younger demographic, especially on a seven day cruise. If you want the most in the way of activity then an Oasis class ship is the better choice.
Princess is fine but it is more laid back and the food is OK. It has been six years since I've sailed Princess. Not because I don't like Princess but they just haven't fit into my itinerary preferences.
Don't forget about the ports. If there is an itinerary that you love then Celebrity, Princess or Royal Caribbean will be fine.