For the age policy of Stack Exchange (a minimum of 16 in the European Union, 13 worldwide).
Due to legislation, Stack Exchange does not allow persons who are younger than 13 to sign up for any site (younger than 16 in the European Union).
If you have question about the age policy or the exact rules governing them, use this tag.
There used to be a "birth date" field in the user's private profile, which formerly populated an "age" field in the public profile, and was translated to the AGE field in the Users table in the Data Explorer. The public profile field was removed after the profile page redesign in 2015, though the private and SEDE fields continued to exist for some time. However, following the passage of GDPR, both of these fields as well as all data related to birth date and age were removed.