40 and feel even younger? I would probably recommend Carnival - also remember the length of the cruise also determines the average age. A 3 or 4-day cruise will have lots of younger people - many of them single. A 7-day cruise will have more couples and families.
Avoid any 10-day cruise on Holland America, and most other lines as well. They generally get almost all retirees.
Me? I'm at that age where retirees don't even look that old to me anymore. The only ones who look old are the ones using walkers or wearing cardigans by the pool.