We're pretty much all had rotavirus as kids. Rotavirus and norovirus are related, but not quite the same. Rotavirus does not mutate and once you've had it, you are immune. When adults do get rotavirus, it's generally mild.

Children are just more susceptible to everything than adults are. Sometimes they don't have a contagious bug. Some adults are also naturally resistant to norovirus, by blood type and secretor status and just strong immune systems in general. Some people have crap immune systems and pick up every stomach bug, flu, and strep infection going around and some adults just never tend to get sick. I know an emet way of thinking (I think this way too a lot) is that if we are exposed, we will get sick, no matter what. But our immune systems can fight it off without us knowing it sometimes. Yay immune systems!