You can do this by applying client validations to the page manually. The first thing you would do is apply a required not blank validation for the checkbox for over 18. Then as responses to this validation you can apply the conditional validations for the other fields that would come next.
There is a bit more information about manually applying the client validations to your form in the help documentation that is available at Help > WebAssist > CSS Form Builder > Help.
If you have any troubles with any part of this or have any questions please post back and we will be glad to help you out.