How to conditionally show error message on the form?

I have 2 date fields successfully working to determine the age of a customer.

However, if the customer is under 18 years old I want the text to show they are underage and hide the submit button. I can work out it all expect I don't know which condition/calculation to use to display the underage text.

You can achieve that by using Text field:

I created condition that will show that text if a person is younger than 18:

And this is how it works:

Here is my form if you want to check how it works by yourself: https://form.jotform.com/62813023149955

Hope it helps. 

If you need any further assistance please let us know.

Thank you!