Do you have a registration form and require visitors must aged 18 years old? Does it look a little comical if the form ask visitors enter their age themselves even there's already birthday date field? In this case, you need to calculate age by birthday and then ask require the age must more than 18 years.
Like the below screenshot the plugin will calculate age by birthday( dob ) and then show error message if the age doesn't match your setting.
For more details about use the plugin please visit Custom Validation Example
Have a try of calculate age by birthday here.