Most succesful racing drivers have started racing in karts ever since they started walking. People like the schumacher brothers, current F1 champ alonso, and the upcoming scott speed are these people (plus a lot more). Then there who those who start a bit late, like jacques villenreve, who attended garmmar school and then i think high school before starting in karts. This is very very race case, and even though he did make it mostly on his own hard work, having a legend as a father did help him a lot. Then theres the near impossible case of Sato, who started racing at the age of 19! What helped him jump into the sport was of course his connections and his ability to get good sponsorship, and plus Honda. What also helped him was that he was in good shape, he raced bikes (not motorbikes..regular bikes) in his teen years so that probaoly kept him in good shape.
So basicaly to sum it up:
Younger = Better. Best if u start around 7 or 10.
Money = Must. They once said, in order to make a small fortune in racing, you need to start with a big fortune when you start racing.
Perseverance = Must. Most racing drivers were damn broke when they started off. One of the richest and most successful Nascar drivers, Jeff Gordon, moved from california to indy when he was a child with his father so he could get strait into the racing world. In those few years he had to sleep with his dad in their pick-up truck since they coulnt afford to rent a place to stay since everything was spent on preparing the karts and buying racing stuff.
Talent = DUH! Comon, goes without saying. This is also why people start so early, so they can know everything about racing.
Sponsorship In the begining, it will be impossible to get a sponsor. In karting? Forget about it. Only possible sponsorships back there is the stuff like Red Bull Driver's search. Then when u work up to lower class open wheel series, you still would find it hard to get a sponsor. No big companys are going to sponsor you since they could just sponsor a F1 team if they wanted. You then need to find a smaller company with intrest in motorsports but cant afford a F1 team for sponsorships.
The better the shape your in = the faster you are on track When was the last time you saw a fat guy win races or become succesful? You need to be in execlelent shape to make it in the racing world. Its not surprising that succesful IRL driver Tony Kannan rides bikes in endurance like tour de france in his off season.
Conections and stuff like that definately help a lot, but it is not always necessary, a lot of the upcoming and current drivers dont have that many connections, but yet they still made it.
Also, if you are entering racing to want to become rich, give up right now because you are just going to just waste your time going nowhere.
Finally, here is the usual path of a F1 driver:
Karting -> Shifter Karting -> Formula BMW, Formula Renault, Formula Nippon -> Formula Renault World Series, GP2 series (this is where the competiton gets professional and even more difficult, it is also where you start to see if you can really make it to the top and where teams start to notice you if your good.) -> F1 (hopefully)
In the beginng karting and formula classes, it will be near impossible to get sponsorship. So you will end up haing to pay for most of those rides (i doubt any team hires a driver...its the other way around untill the big time.)
To become a lemans driver, it would take less prepartion. A year ot two in the first bunch of formula series then a year in the 2nd buch would be good enough. You just always need to be knocking on everyones doors.