You could set the minimum/maximum year for the date field to ensure the user is at least 18, they wouldn't be able to select anything less then that. Aside from that there's no custom validation methods for the date field.
You could try using CB Redirect incubator project and the on before registration trigger, condition the field, if it doesn't match appropriately then redirect back to registration. No idea how well this will work, but if you use the on before it'll stop the user from being created allowing you to have custom validation rules.