Datepicker on form needs age validation

I spent the better part of my weekend trying to find the code needed to require my Date of Birth field, which is a datepicker, to be a date that is age 18 or older. I even went to chatgpt and it printed me out some nice code but alas, it did not work. Is this even possible? I read you can add some javascript in either an html code box or inject it into the header/footer to make this happen.

Might anyone be able to help me on this?
https://www.thehavensrq.org/residential-wait-list

Thank you,
Bruno