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.
I have the same opinion as Shark, there is no age to stop by reading mangas. There are mangas for all ages and even that which is for teenagers or yound adults, in theory, is still funny to read after those ages. In reality, there is a nonsense preconception regarding mangas, since they are like any other book and give the same entertainment.
Manga covers a broad range so even if something doesn't appeal to someone now, they could enjoy it a few years later.
I don't think I would ever stop reading manga, but I'm pretty sure that I'll read less than I'm reading now (and it's not a lot to begin with either). One of the places I would love to work is Japan so if I were to move there, my reading list will be cut down because I intend to read a series solely through a magazine or a tankobon. Since they cost money, I'll only pick the manga I'm really passionate about.
I guess it depends on when One Piece will end
I don't have any particular goal of "when I would stop reading manga" in mind, considering I don't think that Manga only caters to specific age range. Like what have been said by the others, Manga is akin to a book, you can pick it up anytime and if you still enjoy it, it doesn't matter what your age is. I know a relative of mine who has been enjoying Manga for around 20 years - he's in his 40s now. And nope, he's not like super geeky or anything.
It changes with your age, but it changes with "your age in manga" as well. At least with me I began to read simple and famous mangas; even though I still read them I start to read more seinens and more complex stories currently.
I think your taste doesn't change, it is not the right way to say, your taste and interest grows.
I want to read manga along my entire life if possible.
Not only until One Piece finish.
When I die.
I suppose I'm atypical in that I only started to read manga in my early 30's. And I really only did it to continue on with the story of an anime I really like. I got to say I felt a bit awkward walking up to the counter at the bookstore the first time with manga in hand, but now that I've overcome the initial awkwardness of it, I hope to expand to some more series which is one of the reasons for coming here. I'm not too familiar with all the terminology, or classifications, but I do find myself more into the more serious animes/mangas not so much with all the ones that are made for younger audiences where all the main cast are all high school kids if you know what I mean (though there are still plenty of those I did enjoy).
I'll stop reading manga as soon as I can no longer find manga that interests me. Simple as that.