By creating a Age Group Custom Field you would be able to collect your subscriber's Age Group through your Subscription Form.
Please note that the custom field is global and will be available across all lists in your account.
To create a Age Group Custom Field, follow the steps below :
Click on "Custom Fields"
Click on "Presets"
Click on the dropdown and select "Age"
Now click "Copy".
Your age group custom field has been created, now when you create your subscription form you can add the field to your subscription form to collect your subscribers age group.