I am a 56-year-old adult male with Noonan Syndrome. What, if any, effect might the condition have on me as I get older?

In general terms, the main issue is the narrowing of the pulmonary valve in the heart in childhood which may recur again after treatment later in life. There are a number of rarer complications that may occur, but as you have mentioned no symptoms they are unlikely. The rare complications include swelling of the legs, problems with the gut, weight loss, and disturbances in the heart pumping or heart rhythm.