Reports - Age
The Case Data > Person > Age at Intake field will display an applicant/client's age based on their Date of Birth and Intake Date.
The Current Age field on the same table will display their age as of the run date of the report.
If you want yet another option, add the person's DOB field and format it to Age (in Years).
For other people on a case, the same is available on, for example, the Case Data > All Household Members subtable.
Somewhat related: Any date column can be formatted with one of the Age formats to get the 'age' of records. For example, add Date Opened to a report, format it to Age in Days, and it will display the number of days since a case was opened. NB: A filter will not do what you hope and dream it will; in other words, it will not filter on the numeric number of days.