There are a few reasons why a site does age verification:

• Due to the nature of content, the law requires is
• Marketing / demographic purposes
• both

This tip doesn’t refer to porn sites. Over there is a simple 2 buttons solution: Are you 18+? YES / NO

This tip is useful for sites  that fall under the 3rd category, sites that require age verification for both legal and marketing purposes (ex: game websites)

There are 2 ways you can do this:

1. Dropdown boxes
2. Text boxes

The correct answer in this case is to use text boxes. This is so because by using dropdowns your demographic data will be false. This happens because in general users are too lazy to go through a scrolling list and identify the correct values. They will just scroll enough to be the legal age and that’s it. If you use dropdowns, don’t be surprised if the web marketing department will say that your latest gory shooter is played by persons aged 50+.

By using text boxes, users are forced to input some data and in this case they just too lazy to lie about it and they’ll input the correct values.