I think a lot of it is parents would rather their young children be playing educational games on leapster or their ipad/iphone than those kind of Nintendo games. Nintendo could maybe make some more money in the toddle crowd by having some Mario Edutainment type games.
Ha! Before I had kids I thought the same thing but those games blow (they can tell) and kids learn a TON on their own (with guidance, of course). At that age video games are a treat and purely for fun, IMO.
And gaming skill varies by kids. I have a nephew thats 4 and a half or so, and he can't handle NSMB, Mario Kart, Mario, Lego Batman and Sonic Olympic games etc. that i tried to play with him over the hollidays, but other kids can for sure. I think he's just too hyper and doesn't have the patience/attention span to learn the controls etc.
I think 4 is a good age to start introducing video games and skill definitely varies by kid - my son is about 4.5 and he is already sick at video games but a lot of his friends are awful. He can finish Lego Batman easily and he is playing through a bunch of the other Lego games right now (Harry Potter, Pirates, Clone Wars, etc.). He is decent at NSMB but that game frustrates him once he gets past a few levels. I love watching him play with the Xbox controller - he hits every button without looking like it is second nature. Crazy. My daughter (7), on the other hand, hasn't shown as much of an interest in video games - she will play them but she doesn't really ever ask to do so. My son is miles better than she is at them.