The fact is, no 2 PKUers are alike. How each responds and is able to function is dependent upon multiple factors, mostly the specific mutations they carry and their dietary regimen, both as children and as adults. However, even knowing those things doesn’t guarantee a specific outcome. I know my mutations, one mild and one standard (if I remember correctly), and I was off-diet from age 3 to age 32. By that information, one would assume that I’d likely have problems due to high phe levels. However, for whatever reason, I’ve been able to function completely normally off-diet, including holding down jobs (more than one sometimes) and working my way through grad school.