Jackie, on Princess you will find a diverse mix of all ages. True the shorter cruises will attract more young adults and family cruisers over the longer cruises, but its all about you and what you make of your cruise experience. I mean you could meet some fuddy-duddy twenty/thirty somethings just as easily as you could meet some great 40+ year olds. Pick the cruise that best suits your personal needs, wants and desires. Go back to the main page and read Anne's reviews of the ships you are considering. Her reviews are profession and un-biased.
Princess ships and her passengers have a casual-elegance about them, they aren't flashy, or glitzy, but they aren't snooty either. Princess is like a nice resort hotel.