Carnival is the best bet, majority is under 40. I've been to 18 cruises and my
last 6 was on carnival. Have lots of fun on all carnival cruises and met lots of
other singles on board.
Once I was assigned to a table of all couples for dinner. I asked to switch to
different table with singles. No problem at all, the next night I was seated with
a table all singles.
I made 2 bookings with Carnival this years already very reasonable fares.