They could pick a long one. We have just been on a 38 night cruise with Princess & with that length of cruise, it virtually wipes out those of working age.
If you only want to be with those of the same age group then the answer is Saga.
All cruises, there is something for everyone on all of them, read the reviews on here and see the age group the author is in.
I disagree with the 'all cruises' statement - I would never suggest that any one goes on Carnival..
Not really, my parents are cruising for the first time and prefer an older crowd.