Hello kaylin mick, you are not going to find any particular age group exclusively on any given cruise line. The ultra-luxury lines and the very long cruises attract the more mature cruiser mostly because they have the time and the resources to afford such cruises.
But on the mainstream lines, and by that I mean Princess, Carnival, RCCL and NCL, you will find a wide range of ages. If anything, Princess might have a slightly older customer base because their cruises tend to longer. But on shorter cruises, say 7 day Mexican Riviera or Caribbean cruises, I doubt if you would see much difference in age group between the main lines.