I don't have exact demographics, but most Princess cruises seem to have passengers of all ages. The median age might be slightly over fifty, but half of the passengers are younger than the median age. Thus, it's a very misleading statistic. You probably will find plenty of passengers around your own age with whom to socialize. Activities like the beach games on Princess Cays and some of the pool games are excellent opportunities to meet other young adults.
Have a great cruise!