By creating a Age Group Custom Field you would be able to collect your subscriber's Age Group through your Subscription Form.
To create a Age Group Custom Field, follow the steps below :
Click on the "Lists" tab.
Select your list.
Click on "Custom Fields"
Click on "Create New Custom Field"
5.1 Give your Age Group field a name.
5.2 Select "Drop down" in the "Type" Section drop down.
6.1 Insert the age groups.
6.2 Click on the + sign if you would like to add more labels.
6.3 Click "Create Custom Field".
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.