P and O; Princess and Celebrity. Avoid Carnival tends to younger age gropus. Again the time of year has its influences with work and school the middle aged cruisers [UK] have to stick mainly to June/July/August and maybe early September. The Americans and Canadians want to escape their cold weather so November to March for them. So looking at this logically April/May/early June and possibly October might be your best times but with cruising still a major growth area it is a bit difficult to predict.
I agree it depends where and when you cruise we have cruised twice with princess, fantastic ships. we did caribbean in january last year we are 50 an 51 and we were amongst the younger age group,mostly english mostly peop;e in 60s a few were younger but mostly older but we had a fantastic time, the caribbean is fab. we did a med cruise with princess in june this year mostly americans but a wider range of ages, family's with young kids and teenager's (they were well behaved and no problem) groups of friends of all ages and older people again we had a fab time.