Can somone with PKU go undetected past their childhood?

Let's say an America, typical American diet, about 15-20 years old, would it be possible for them to have PKU and not know it?

Without treatment, most infants with PKU develop mental retardation. Those with untreated PKU may also develop additional neurologic symptoms.
Babies born with PKU in the US and many other parts of the world today are identified in infancy through newborn screening programs. They are immediately started on a PHE-restricted diet. Early diagnosis and treatment prevents mental retardation and other associated problems. Many early-treated young adults with PKU are now in college, or have completed college and have productive jobs. In adults who are untreated, the major goals are positive health, and emotional and behavioral stability.
So if you are 15-20 and asking this question, NO you do not have PKU. All the tests at birth and physcian screenings during child hood it would get caught in todays society!
Good-Luck =-)

No. Probably not.
They would have been severly retarded and have died from the toxic build up by then.