River cruises undoubtedly are targetted at Baby Boomers with average ages being between 55 and 70 despite the brochures often showing guests much younger than you would regularly encounter. As has been said, many river cruiselines actively discourage children though a few are running special family cruises during school holidays but families need to be aware that the ships do not contain dedicated children areas and many of the destinations may not be particularly exciting for children.
If you are trying to find a slightly younger crowd, you should probably look at the 4 star lines rather than 5 star. Some cruiselines have two brands within their stable such as APT (5*) having Travelmarvel (4*) and Scenic (5*) having Emerald (4*).