Custom message for checkbox

I have a check box field in the contact form that is required:

<p>Select Age (required)<br />
[checkbox* your-age use_label_element "Under 13 years of age" "13 years or over"]</p>

At the moment, as long as the writer selects a checkbox the email will be sent. I want to change this so that if the writer selects “Under 13 years of age” an error message appears, e.g. “Please ask your parent or guardian to relay your message” and for the email not to be sent.

Can anyone suggest how I could do that?

• This topic was modified 8 years, 11 months ago by ozviewer.