It is not possible to make sure the player is not lying on its age when filling the form.
However you can activate some option to make the player understand people under 18 cannot play.
- In your form add a mandatory checkbox like "I certify being older than 18"
- Limit the date of birth field. When creating a field "birthdate in your campaign": choose the field "date field (dd/mm/yyyy) with calendar". Then click on "validation" (just above the type of field). You can choose a "start year" for the calendar. Depending on the current year and the minimum age you want player to be : choose the date of start for your calendar.
- Always add in the rules of you game the minimum age required to play, and that a justification can be asked.