As there is a natural tendency for evolution(for good or bad) in almost everything, the younger generation may actually be smarter than the older generations mostly on an average scale. Again this cannot be generalized as there can be exceptions.
I don't think anyone has understood the source or origin of the prodigy/genius characteristics in a human being. Again the genius thing is mostly restricted to some 'area' or 'field' only and probably brings down the level of some other characteristics(absent-mindedness, eccentricity etc.). Almost makes me think, it is akin to the distribution of a fixed number of 'experience' points among the various attributes/skills of a playable 'character' while starting a game, which we then go ahead developing them. The more you spend your resources on (or) train a particular attribute, the more it develops. Whatever that governs(or not governs???) the working(???) of the Universe is doing a similar thing with us or what. Alternatively it might be just the continuity of the knowledge/wisdom from previous births(where am I getting here?).
Crazy thoughts aside, on a larger scale, the increased smartness might be due to the readily available inputs(huge) in the form of media(Web mainly) which we didn't have or only had at a later stage. This is not to belittle the younger generations as I had been taken aback few times(the catch is, not necessarily because they are younger than me). However IMO, I can't accept that my relatively older generation is in anyway inferior to the younger generation.