The table should actually look more like this:
VisitID PatientID Date Diagnosis Treatment 123 25 18.01.2011 flu lots of fluids 124 25 19.02.2012 migraine painkillers
and it would be related to the table of Patients as:
Patients::PatientID = Visits::PatientID
If you place a portal to Visits on a layout of Patients, you'll be able to create new visits directly by entering them into the first empty row of the portal (provided you enable this in the definition of the relationship).
All of this is pretty basic stuff and very far from impossible. Note that the age of a patient at the time of a visit can be calculated from the visit's date and the patient's date-of-birth (stored in the Patients table).
and THAT'S how the magic works! Thank you SO much!