Yeah, 42 is not too old to be playing. Simple fact is that there isn't an age that is "too old" to play Pokémon. The store owner really shouldn't be making those kinds of comments in my opinion, because it's not like your age is something that actually matters while playing. Regardless of who you are, or how old you are, Pokémon is something to enjoy that everyone can enjoy. I've met parents of people I've played the TCG against who are at least 50 years old and they still enjoy playing the game.
You've just started? Have fun, man. And welcome to PokéBeach. I hope you enjoy the game and your experiences that come with it, experiences that will last your lifetime, regardless of your current age.
~ VioletValkyrie