The doc page says that the
user object returns an
age property (for those users that specified an age).
- Age was removed from users' (site) profile pages in April 2015.
- Network-wide profile pages dropped the Age display in April 2017.
- Sometime in the last few years, the site stopped allowing you to enter your age.
- The Data Explorer (SEDE) stopped reporting Age yesterday (June 2018).
Update: Nick Craver says that the
age property cannot safely be removed, but that it will now always be null (which means that it will not be present in any API results).
So, from the user's perspective it is removed, but it will still be present as an obsolete option for filters.
The documentation still needs to be corrected.