I'm aware that in 2018, the GDPR was adopted in the European Union, and in response, SE amended its license terms to require that all users from the EU be at least 16 to participate in the site. Previously, the age requirement was 13 worldwide (due to U.S. law).
What if, for example, someone created their account as a 13-year-old in 2017, fully compliant with the existing rules at the time, but suddenly found themselves being declared "underage" once the change took effect (i.e. would be 14 years old in May 2018, and 14 < 16)?
Were they required to leave the site and have all their information removed, just like how underage users are handled today?