You can set a minimum age on a menu. This will prompt your customer to select if they are a specific age+ on the checkout page.
Login to OrderCloud
Select Manage Stores
Select the Location
Select the Menu to set the age restriction
Select the Menu Settings tab
Under the Settings section type in the age to show on the checkout page
How it shows on the checkout page
Please note, that even if a customer doesn't have an Alcoholic item to purchase, they will still be prompted to verify Age during the check out process with this setting.