That's working as intended. 0 is no age so it's going to include users with no age. A full range search is treated as no search since there's no other way to clear that search criteria. Only way to workaround this is something like the below to allow a wider range search.
Minimum Year: -100
Maximum Year: -14
Minimum Age: 18
Maximum Age: 100
It has always functioned this way, but it really feels like buggy behavior. It should ideally just have the "No Preference" option to signify no search like other select fields have when searching. Have added a bug ticket for this.
forge.joomlapolis.com/issues/8141