Check out some websites and see which lines cater to your demographic. Some are geared toward the geriatric set, others are luxury lines, etc.
That is true and it also depends on the time of year.
I have 5 (soon to be 6) cruises under my belt, all on the "geriatric" Holland America Line. The vast majority of passengers on each of my cruises (but one) have been in the 50 - 60 year old range (hardly geriatric).
The exception was the cruise I took over a spring break a couple of years ago. The demographic was a lot younger with teens everywhere. Even then, I have never run into loud drunken parties, passengers throwing up in the hallways or doing unspeakable things in the pools - all of which I have heard about on lines like Carnival, Norwegian or Royal Caribbean.
I would recommend checking out websites like the cruisecritic.com mentioned in the article. There are sections for each of the major (and minor) cruise lines where users post threads, etc. I often see threads asking for someone to "Convince me why I should sail on this line", etc.