Can an individual with pku stop following the special diet when he is older?

Depends. Although the guidance 30 years ago often loosened strict dietary restrictions as early as 5-6yr, later work has shown the negative effects of this on personal performance throughout life. Women have additional burdens as their pku can cause brain injury and malformations in her baby if she is not on a strict diet for months before conception & throughout pregnancy. Stop & u will pay a price.