I've seen a few folks looking around for something like this. I have an age limit of 16 on our forums. While this isn't an actually file you can click and download and here you go. It's very simple to do, and will take approx 10 minutes.

In your forums location /core/vb/library there will be a file user.php
Do a search for 'needsCoppa'. You'll find a public function near line 753.

Look for: ........

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Original Source: Mini Mods - Simple Age Restriction for Forums