I'm not harsh at the companies for marketing to kids, that's a legitimate business strategy, see my praise of Frozen. I'm stating that it's a difficult business strategy since kids grow up. A specific kid will have an evolving age with time, and a specific age bracket will have an evolving population of kids with time.
You can argue that this has been true for the comics for 70 years, but in the case of the comics there are hundreds of comics being printed every month, which allows more experimentation and diversity within the medium, so old styles can organically die and be replaced by new styles, leading to the different comics "ages" transitioning in a relatively seamless manner. In contrast, movies made at 200 million a pop do not afford much room for experimentation, and thus there is a risk of the market evolving beyond the product.