How I can put required value for a field?

Hi I just make a form that calculates the age of a person and I want to have a condition there if the age is less than 18 then the form would show a require value must be above 18, or prevent the user from continuing, I am using the new form layout.

You can make some other field required if the age is less than 18 by using the Required conditional logic:

You could also use show/hide condition to show a certain message for example if the age is less then 18, or hide the submit button page:

Hope it helps. 

Let us know if you have any further questions.

Thank you!