You are going to see a mixed crowd on the Lirica. Many international passengers, as well as all ages of Americans. A great choice for a couple in their thirties would be Royal CAribbean followed by Carnival. It also depends on when you cruise and the itinerary, as well. Cruises ;pmger than 7 days tend to draw an older crowd, as do Panama Canal and Alaska cruises.
You will have fun no matter what, however. Enjoy the Lirica.