Hi,
Thanks for using our plugins. Sorry for the late reply. I can try to help out here.
We are limited in providing support on customizations so not sure how that could be done on the checkout page.
What about using our new attendee meta and creating a checkbox that states they confirm the ticket is for an adult.
They would have to confirm per ticket, but the process to set that up takes minutes versus hours for a custom solution that we are unable to provide support on.
Using the attendee meta you can create the fields for that and reuse them on any “adult ticket”.
Here is more about that feature:
https://theeventscalendar.com/knowledgebase/collecting-attendee-information/
Does that help?