Set up Item to Verify Age:
Navigate to Configuration > Items and select the item you wish to modify. Click "Edit" to edit the item. Select "Verify Customer Age" and enter the age limit.
Tip: You can set this at the category level, then new items in that category will default this setting.
While you are in Configuration, you may want to check the Security settings for this feature. This is in the 'Manager Functions' section.
In the Point-of-Sale, when you order an item with age verification activated, you'll see a pop up asking you to enter the customer birthday.
When the birthday is entered, it will be recorded on the ticket. If someone with proper security uses the "Bypass" key, that will also be recorded.