Not sure about all gay cruises as I have never been on one, but you will find many younger people on most cruise lines these days. A lot depends on time of year you are traveling and the itinerary. My partner and I are in our 40's, but there were a lot of younger people on Holland America on our trip to Alaska this summer. I know HAL gets the rap as the senior line, but this was not true in Alaska. We also met a lot of younger people on Carnival on a cruise to Costa Rica and Belize.
Two years ago we took a 4-day cruise from Los Angeles to Mexico on Royal Carribbean over Spring Break and I would say approximately half the passengers were college age. This would probably be true for most Spring Break cruises.
The average age of cruisers has definitely moved down over the years. One way to meet younger people is to book the more adventurous shore excursions i.e. snorkling, hiking, zip lines, etc. Another suggestion is to look for the newest ships that have rock climbing walls, bowling allies and other activities geared towards younger passengers.