A cruise is what you make of it, don't get stuck on the whole age of passengers issue.(Example - our first cruise- we were in our early thirties traveling with 2 other couples in their mid- twenties, we chose a 10 Day Panama Canal total transit on the Royal Princess - we were the youngest people on the ship(most were 60+) did that stop us from having a wonderful and memorable cruise, heck no, we chose this particular cruise because of the cruise line and ports, and to this day that particular cruise has been the most memorable for us.)
Go to a local travel agent, one who specializes in cruises and has taken a lot of them. Tell them what you interest are, what you like and don't like to do, what your hopes and expectations of your cruise experience are. One of the first questions i would ask you is, are you looking for a "traditional" cruise experience, that old-world style of elegant cruising - there are lines that are known for this. Or are you more interested in a "casual elegant" type of cruise experience, more like a resort hotel, with activites, day and night, sports etc and great service? Or are you looking for something more Las Vegasy with all the glitz, glamour, bells, whistles and stuff going on? Narrow that down and then pick a cruise that goes to ports of call that have things that interest you. Its a lot of information to digest, but a good TA will help you sift thru it all.
Another thing you mentioned is that you don't want a cruise with lots of kids on board. Then stay away from school break, and summer time.