For the age policy of Stack Exchange (a minimum of 16 in the European Union, 13 worldwide, 18 to be a moderator)
Due to legislation, Stack Exchange does not allow persons who are under 13 years old, or from the European Union and under 16 years old, to sign up for any site. Additionally, users under the age of 18 are not permitted to become moderators on any site.
If you have question about the age policies 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.