Custom Code For Age Restriction

I have a blog on wordpress.com that I’d like to make age restricted (18 and over).
You know, so you have to check a box that verifies you’re old enough to look at a curse word or comics occasionally depicting violent or graphic images.
I wrote to support and they said the only way to do this is to make a wordpress.org site and create custom code for the popup box.
I have no idea how to code that.
I googled the question and didn’t find anything.
Has anyone already done this for their site, or know where I could find instructions on how to do that, or are willing to help me with this?

I would be SOOO appreciative.
Thanks!
–Malkinskin