When you mention the word "Pokémon" to a non-fan, then the first thought that comes to their mind is the little yellow rodent saying "Pikachu" and the goofy kid with a hat. Why is this true in the majority of the cases? Because the anime show was really the aspect of the franchise that received the most attention. With the seizure episode for Porygon, the movies becoming blockbusters, and the show on television most of the time - getting its start on basic cable, which enabled it to be viewed by the widest audience - it is no surprise that most people know about the show.
The plots of the episodes do get a little stale over time, with the same basic formula being followed for most of them. But, then again, the only thing on the writers' minds is advertisment for the latest game/movie/money maker. If you want any real character development, deep plots, or anything of that sort, you are watching the wrong anime.
This is an anime for little children. All that they want to see mostly is their favorite monsters in great battles. They really don't care what sort of emotional development Ash went through as he loses a battle. They just want to see Charizard kick some major...gluteus maximus. (There are exceptions to the rule. I know that there are some young children who can follow a complex plot. More power to them, I say.)
The best thing to do with those who say that the Pokémon franchise is for little kids is this: Show them the games. There are the EVs, the IVs, the GVs, and other strategic aspects that would make them think twice about calling Pokémon "childish." And if they still don't believe that there is something for every age group, then really, there's no hope for them!