Hi Frase, HAL typically caters to an older demography, but....I cruised on them in my late 30s by myself. I had a wonderful time. It was my first HAL cruise. I met two ladies from Canada that were single and in my age group, and a bunch of couples that were in their 30s-40s. I did a Caribbean out of New Orleans on the New Amsterdam. It was in the early mid 1990s. The food wasn't as good as some of the other lines, to my taste only. The ship was immaculate, the service was great, the cabin was a nice size (I had an interior), and they offered some nice ammenities on board. The casino was tighter than on most cruises I've been on, but their tight on any of them.
I did cruise again with them in 99 or 2000, (can't remember) on the Rotterdam and thoroughly enjoyed it, still didn't think the food was that great. We all know that food is totally subjective. In fact, I was thinking about doing a 3 day out of Ft. Lauderdale, just for a nice weekend, they send me great email offers.
I don't think you'd be disappointed with HAL, it just won't be very lively after 11pm. Have you tried Costa or Princess. Costa is more like Carnival, Princess is more like Royal.