Visit validation should fail if the startDatetime is before the patient's birthdate.
If the patient's birthdate is estimated, then we cannot strictly validate against the exact value of patient.birthdate (because the OpenMRS API should be pretty permissive, though specific UIs and forms can choose to be stricter).
- Darius's proposal: if patient.birthdateEstimated is true, then add a 50% margin of error to patient.birthdate before validating, adding a minimum of 1 year. (E.g. if a patient is approximately 20 years old we would allow visits up to 30 years ago. If a patient is approximately 9 months old, we'd allow visits up to 1 year and 9 months ago.)