Is pulm valve stenosis usually (almost always) diagnosed in child hood?

Most of the time: Most of the time yes and due to congenital heart disease, there are conditions like carcinoid syndrome and rheumatic fever that cause adult pulmonary valve stenosis. These diseases are symptomatic and affect more than one valve.

Yes: Typically, pulmonary valve stenosis presents with an asymptomatic heart murmur, often associated with an ejection click. Most patients are diagnosed in the first year of life.