I haven't actually run into that, but if I did, I would ask them if they've been living under a rock, promptly tell them that millions of people my age and older play Pokemon, that all the main series games ALWAYS get awesome reviews, that they are some of the BEST selling games of ALL TIME, that Diamond and Pearl ALONE sold over 16 million in just 2 years, that all the main series games COMBINED have sold over 75 million copies worldwide and that there is only one reason that could be: it is a damn good game. A classic. One that is surprisingly complex, and suitable for all ages.
...Yeah I know it's a long answer, but it would get my point across. :b They should learn before they criticise, lol.