Cross-protection from seasonal epidemics of human coronaviruses (HCoVs) has been hypothesized to contribute to the relative sparing of children during the early phase of the pandemic. Testing this relies on understanding the prepandemic age distribution of recent HCoV infections, but little is known about their dynamics. Using England and Wales as a case study, we use a transmission model to estimate the duration of immunity to seasonal coronaviruses, and show how cross-protection could have affected the age distribution of susceptibility during the first wave, and could alter SARS-CoV-2 transmission patterns over the coming decade.