Just want to give an updated answer, because Shopify Checkout Extensibility has made the current answer stale.
In the future, you won’t be able to modify checkout.liquid, Shopify has replaced this with app blocks that you can drag and drop into your checkout page within your theme.
It’s extremely simple, and we’ve build an app block that verifies government issued IDs such as drivers licenses, passports and/or national IDs (including AU and NZ).
Here’s a short video that shows how to set it up on your store:
And here’s a link to our documentation on this Checkout Extension to learn more: https://getverdict.com/help/docs/flows/shopify-checkout-app-blocks
Yes, the app can apply your minimum age restrictions, and it can be white labeled to fit your theme.
If you have any questions, please just let me know! Happy to help.