Hi, I'm 36 and was off the diet for about 30 years. I've been back on for about 3.
I definitely recommend going back on diet. Even if you think you aren't having problems being off diet, it's worth being back on.
I am on Kuvan, have been for about a year, and I've gotten my levels from around 12 prior to Kuvan to well below 6. My most recent level was actually a 1.6.
Also, one of the big issues that has been discussed recently is the increased likelihood of osteoporosis in PKUers. Off-diet, apparently low bone density is common, and on-diet, PKUers often don't get enough calcium.
I just had a bone density scan yesterday (should get the results tomorrow), and started a calcium supplement today.