Infection with at least some of those "common cold" human coronaviruses seems to have much the same effects on kids and the elderly (and maybe everyone else too) as Covid-19. At least part of the reason we're not seeing a huge pandemic of those is that lots of people already have immunity, but that also makes infecting people with them less useful - the people who already have immunity to those aren't likely to develop new immunity, and the people who don't presumably won't necessarily develop immunity that crosses over to Covid-19.

(Unfortunately, since common cold research is a lot less big, glamourous and high-profile than Covid-19, I don't think we know a huge amount for sure about the other human coronaviruses - and what we do know isn't exactly front-page headline news.)