I've been on both Celebrity and Princess, though only one of each. Both had piano bars, though no one sang along on the Princess one. I think that had more to do with the particular pianist than the ship. My experience is that the HAL piano bars are the best.
Both ships carry enough passengers so that you will be able to find a reasonable number of people in your own age group. Just go to the things that appeal to you and chances are, they'll be there too. People on cruise ships are invariably friendly; you can probably pick out someone who looks about your same age and get acquainted as early as the sign-in, or in the cafeteria for lunch your first day.