Why do most people stop giving handjobs after a certain age?

INTERESTING! Not sure how to answer this one. I would say to each there own maybe as you get older you can engage more easily in other forms of sex. Some younger people think that intercourse constitutes sex and other activities do not.
Can't find research- On that specific subject and honestly have not had much information from patients about it over my 8 years of general practice and then 37 years of psychiatric practice. It's obviously a normal, natural phenomenon and i can't think of any particular reason to vote on either side of the question. Tap me back if you have any ideas.