According to today's interview with a NY Times reporter on The Daily podcast, the immunity is explained by the close relationship to the common cold. Children tend to get lots of common colds. Around 1/3rd of common colds are a form of coronavirus, because of this, their immune systems are more prepared for this new coronavirus. Over time, this immunity fades, but it takes many years.

https://www.nytimes.com/2020/02/27/podcasts/the-daily/corona...