Hi,
If you use a custom field of the table "user", the field will only be required during registration. So if someone checkout with an old user account, HikaShop won't check whether that field has been filled or not in his profile.
A custom field of the table "address" will allow for that, but that means that each time the user adds a new address, he will have to enter a birthday.
A custom field of the table "order" would also do that, but that means that each time the user go through the checkout, he would have to enter his birthday.
Maybe you would like to consider something like this instead:
elfsight.com/age-verification-widget/joomla/
That way, it would ask for the age of the user when he accesses the website.