When you enable the Age Verification add-on in your store, any not-logged-in customer must pass the age verification to access the store content.
Depending on the display mode configured for the pop-up, a visitor will be suggested confirming the age with a button or specifying the age manually to access the store content.
Two-button display mode
Text verification display mode
The underage visitors will not be allowed to access the site and will see a respective message with a suggestion to leave the site.
All customers who pass the age verification limit will access the site content.
The age verification form entirely relies on the visitors' trustworthiness and can not 100% prevent underage visitors from viewing the store content in case of deception.