By entering a contact's date of birth into Vertical Change, you automatically have the ability to report on the age of that contact at the time the service was rendered. Here's how:

In the Report Builder, select the Birth Date field to include in your report. This field typically lives in the Person section in the report fields available.  Most agencies put Birth Date in the Common Fields form, but you might also find it in their Contact Sub Type form (like Youth, Parent, Client, etc.).

Once you select the birth date to include in your report, select the filter on the Birth Date column. In the dropdown that will appear, select "show age when form completed" and click DONE.

Vertical Change will create a new column next to the Birth Date column called "age at service" and will show the age of the client at the time of service there.

Once you see their ages, you can filter that column the same way you would when filtering any normal numeric field.