River cruises are definitely NOT just for old people!! There are NO KIDS, but most are middle aged.....40-70....depending on the line. We did 4 Crystal River Cruises and they were the BEST TIME EVER!
That is the demographic they are tying to advertise to. They are expensive, but you get what you pay for, IMHO. My wife and I went on a Windjammer Barefoot Cruise for our honeymoon 28 years ago. It was one of five vessels that motor/sailed around different Caribbean islands. The one we went on was out of Tortola. It did a weekly circuit and stopped at various British Virgin Island destinations.
Generally going where smaller yachts sailed. It was great.
It was also a week long booze cruise. Which was fine for us at that age.
The Euro river cruises are nice. I may go on one in the future. However, I am more of a set your own itinerary type