(You can skip/ignore this paragraph if you want) It seems odd to change the value of Age in PRecord instead of in Patient. It would seem that age and Sex would both be values to define in Patient, not PRecord. And Patient could link directly to Stds, with Precord doing either a lookup or link to that data through the Patient record, but then maybe PRecord is capturing data on Patient at different points in time such that age is not always the same in every record of PRecord for a given parent record in Patient.
But I see no reason why changing the value of PRecord::Age would not change the value linked to in Stds provided that the change is sufficient to match to a different record in stds. Whether with look up or dynamic link, a change in that value should either relookup a new value or link to a different record. You might, as a test, put a portal to stds on your layout so that you can see all related records and then watch to see if the records shown in the portal change when you change the value of Age.