Interesting post to read! I'm past 30 but thought I would comment as I've always been single and I have cruised ALOT!!!
Maryann, on HAL, Europe cruise, everyone is going to be older, possibly much older. I am surprised your TA suggested that, I would have said, Princess or RCI for sure.
Nothing wrong with older (I'm 56) but I'd do more homework first as you have time to change.
I think all the major cruise lines, Princess, RCI, Carnival are fine, you just have to know what you want from a cruise, what you like to do and you can avoid, the drink til you drop crowd. Carnival does get a bad rap, they don't deserve. Personally, I'm not wild about the layout of their new mega ships, but food, service, entertainment ranks right up there with the rest of the mass market.
I've been on 6 cruises in 3 yrs., five different lines and for people to assume Carnival is all drink, drunks, party, party is just plain wrong. My favorite lines are Princess and RCI but they just fit the bill for me personally, other people may see it differently. On a recent fall Carnival cruise (the new Glory) there was a huge variety of age groups and everyone seemed to find their niche.