The generic demographic on river cruises is old. They have no activities for kids. THey have no space in cabins for kids. This is the first year a couple of lines have offered single cabins & connecting cabins and some family trip options (and Disney is partnering with one company). But in general, no, river cruises (note I didn't say anything about barges, or yacht charters either in case that's where you go next
) are not full of kids. I have no idea how this is a stereotype of luxury that you mention, but whatever....
Since the OP wants to avoid kids, it's an idea. I did a river cruise in 2014, age 41, and I hated it. It was a lovely cruise but I'm an independent traveler and I didn't like having to pay for things I didn't want to do (eg "included" tours). We did a Christmas Markets itinerary since I love the Markets, and there were 2 families (youngest were old teens) and a couple of middle aged couples like us, but I would say 85% of the guests were >65. That's neither good nor bad, it's just what it is.