I dont think I could see an age to stop reading manga. It would be like suggesting an age to stop reading books. I thought I would potentially outgrow the young adult novels and what not, but well even now starting mid 20 or so I still read those books as I guess their fun. I dont think Id stop manga either, even this long after high school I guess I still enjoy reading all these series on high school clubs and what not, its just fun to sit back and think of those days or even just imagine myself for a few moments in these series thinking back to simpler and funner times, plus its like a fantasy since manga school is > IRL high school, would have been fun.