Well the first half of what you said is flawed. Your right in that the vast majority of cases occur when we are much younger, primarily when we are young children. However your not accounting for sudden onset stuttering. Something me and thousands of others have to deal with. Take drug use for example (which has been directly shown to cause similar Dopamine activation en masse, with Pornography. There are studies containing PET scans comparing brain activation. So back to my point, if somebody were to take drugs and lose their fluency because of it. It simply isn’t anecdotal. Their is a concrete link between the two. Before that person began drug consumption, it hey didn’t stutter. Your right in that masturbation has nothing to do with a lot of people. However some people have susceptibilities in their gene expression making them more sensitive to dopamine. I can link these studies if you would like. Also masturbation is not the issue, and in fact I never once even mentioned masturbation. Pornography and binging on pornography is what causes down regulation of receptors and transporters. My response to your second half goes back to what I said previously. Some people have variables in their gene expression that gets activated and agitated through sudden changes in neurochemistry like porn, drugs, sex addiction, etc. as well as stress, trauma and other things. So no it isn’t as simple. Somebody can stutter from birth or not stutter at all, and can watch porn all day long and not suffer as greatly as somebody else with different gene expressions in their DNA related to different levels of dopamine then the average person. The goal is to not experiment and even try porn here. Because unless you get your genealogy tested, you just never know.