study with troubling implications
Seems to be of good size and quality.
It would make some considerable sense. It is fortunate that small children don't tend to become acutely ill and (probably) don't spread the virus very heavily. But we also know that young adults do spread it, a lot.
Therefore it is unlikely that the virus has an innate ability to be benign and non-virulent until after the 19th birthday.
Might be an important consideration for people making policy at any level. Also might be a good reason for people who are just blowing hot air to shut up.