Can someone go through 65 years of life normally not knowing that they have pku/hpa and then suddenly at 65 have an onset of symptoms?

Doubtful: There are variants of phenylalanine metabolism.This includes a malignant form that responds poorly to any form of rx, classical pku & those with benign hyper phenylanemia that never were affected.Classical untreated pku results in mental deficiency/seizures/and more by 5 years of age.I'm not aware of followup data on those with benign hyper-phe into that age group.

Very unlikely: Classic pku is diagnosed early in childhood and untreated creates significant morbidity & mortality. The main difference between those who have classical pku and those who have hyperphenalaninemia is that people with hyperphe tend to tolerate more phenylalanine without having high blood levels, so guess it is possible, but unusual.