Age does not include data for users younger than 18 years and these audience dimensions can cause Google Analytics to filter rows because of privacy reasons:
The more dimensions you select and the more specific report you create the more rows are likely to be filtered. Read more about the Data Thresholds here: https://support.google.com/analytics/answer/2799357#thresholds
Mobile Device dimensions
Choosing any of the dimensions below might lead to unexpected total values, as they will not include data for desktop users. This is the same behavior as in Google Analytics' UI and has to do with the fact that data is only returned for rows where these dimensions have values.
- Mobile Device Model
- Mobile Device Branding
- Mobile Device Marketing Name
- Mobile Device Info
Google Analytics segments the data based on which rows has values for the selected custom dimensions. Choosing two custom dimensions means that each row must have some set value for both of these dimensions, if one is missing the row will not be part of the segment of data returned. This is the same behavior as in Google Analytics' UI.