OP is a 25 year-old manga and Naruto is a 20+ year-old manga, though. They probably started reading/watching when they were teens or even pre-teens and simply continued into adulthood. From what I know, most of us active/recurring users on here are at least in our early 20's. Some users are in their 30's and I'm pretty sure there are a couple of dudes reaching 40.
AFAIK, in Japan anime is not necessarily considered "cartoons for kids" but can often be considered a type of literature; most people, even the adults, watch it in some way or form. Even stuff that's deemed as "shonen" often has themes and motifs that far bypass the comprehension level of children.