How common is it to be diagnosed for the first time at age 36 with cystic fibrosis? my son complains of light discomfort in ruq for 18 months, cbc/urine all good. never sickly as child, would it show in bloods? he notices a quick wrinkling in hands?

Cystic fibrosis (CF) is usually diagnosed well before age "36", so unless there is a history of repeated lung infections, and respiratory distress, CF is unlikely. See a pulmonology physician for an exam and further testing.

If you are convinced this is one of the many forms of Cystic Fibrosis, have the kid tested for the genes involved. Otherwise let the issue go since it is not realistic. In medicine we are taught to never say never, however with CF there are available genetic tests that can exclude the label. A clinical geneticist at a children's hospital or medical school can arrange the study.