I'm Jason a Guru here at Shopify.
I would suggest checking out an age verification app such as age check. If you sell any age restricted products on your website, Age Check is an essential app that verifies the age of visitors when they arrive on your website!
There are quite a few age verification apps available in the app store which you can check out here. I would attempt to install a few and see which one suits you best.
I hope this was helpful, looking forward to your response! Have a great day!
Jason | Shopify