In order to verify a customer's age upon buying a certain item, you first have to go to Store Config > Items and Search for "Verify Age For Products". Check this setting first and apply a default age to verify.
You may then click the Edit button to go to the item's Product Information tab. There is a checkbox on the bottommost part of the page, which if checked, would require the staff at the register to verify the birth date of the customer who asks for the item. You will also need to enter the age required for this item to be sold. If there is no age set, the default age to verify will be applied.
Do not forget to hit Save before leaving the item information page.
If the item is going to be sold on the Sales screen, it will prompt you to enter the customer's birthday (MM/DD/YYYY). The system will automatically compute the actual age of the user -- and if his/her birthday is less than the number you have indicated in your inventory info screen, it will get removed from the sales page.
Let us know in the Coreware Help Desk if you have other questions.