I wouldn't say there's an age limit. I guess there's a limit to how much from the universe you accept. I know people that would never consider buying Pokemon Diamond, Pearl, Black, White, and some even Ruby or Sapphire. But if you offered them to have a shot at FireRed, LeafGreen, Red, Blue or Yellow, they'd go "hell yeah, I love those games".
There'll be a time when everyone who likes Pokemon currently will stop liking new products but newer fans will. The older fans will probably continue to play with the "vintage" titles until they get bored of them.