I don't think that is actually the case. Children are far smarter than you seem to give them credit for. I've been helping my younger cousins with their homework, and they seem to be tackling more things than I did at their age. The idea isn't so much to teach them how to drive initially, but I don't see why it would hurt to give them simulations. I mean between Need for Speed, Mariokart, and others, what kinds of things are being put into their minds about driving? You crash, and its okay because you can restart. Not the case with life. Starting young is the best means of changing their behavior towards driving. The only way to tell whether or not this works is to wait 10 years and do studies on them. But I think putting up the money is well worth it.
And I go back to one thing: simulation. Their feet can reach the pedals. These simulations are VERY realistic. You get to drive. You feel the impact when you get hit (the entire ride shakes). Think of the ones you see at an arcade when you sit down and so forth. If you put forth rewards of driving at certain speeds, acknowledging street signs, and everything, they will be more than ready when they actually do get their licenses. Dealing with full blown puberty driven teenagers is very difficult. Teaching them to care and realize not to speed and to drive safely is actually fairly hard. They're receiving a sense of freedom, and 6 months is not enough to prepare them for a life-time of driving IMO with others on the road.