JoomlaXTC Age Lock and Hikashop

Hi,

That's because the code of that plugin is added inside the HikaShop product page form on that page.
It should be added before or after but not inside. If it's inside, when the button is clicked, it will trigger the add to cart system.

I don't know how you added that plugin inside the product page form. If you did that yourself, then it's a simple matter of moving your code outside of the form.
And if it's done by the plugin itself, it will require contacting the developer of the plugin for help in order to change the code of the plugin in order to take that into account.