Adding a custom JQuery Validation

Hi! I’m new to SuiteCRM and I would like some help with adding custom validation. So far, I’ve only added the js file via ‘includes’ in the editview. My goal would be to show an error message whenever the date entered is below 18.

I’m currently stuck with how will I target the div below the inputs where the errors are usually shown.

My english isn’t that good so I’m sorry if I’m not clear.
The TL;DR would be:
I input a date > shows an error if age(calculate from birthdate) is below 18. The displayed error would look like the default error message if, say, I inputted a text in an int field.

You should have a look at :

Thanks for the link!. I have read it but have no idea where to put it or which file should I specifically insert it :frowning: .