Online websites that sell age-restricted products or services need age verification. The businesses selling age-restricted products had to comply with age verification regulations of the jurisdiction they were operating in. Usually a type of this website use the Claspo tool for it, you can read about it here: https://claspo.io/blog/age-verification-pop-up-why-do-you-need-it-and-how-to-add-one-to-your-website/