My son has been doing it for about three and half years now. He started in a kid kart when he was 6.
Matter of fact, we're in Palm Beach for the Florida Winter Tour this weekend. Yes, we're serious about it.
Couple of points:
1) Make sure they want to do it, not dad. I've seen my share of dads screaming at 5 year olds....
2) If you can, find a track that offers rentals. We did for six weeks and when he was begging me to go to the track three times a week, I knew he was serious and bought him a used kart
3) Buy used to start off. Kid karts are the way to start. When they get older and outgrow them, you'll get what you are running locally.
Karting CAN be expensive. Kid Karts are probably the worst about spending money, because dad thinks "just spend more on motors rather than teach Jr. to drive."
If they like it, they'll want to do it a lot. If they like it a lot, they'll get lots of seat time. Seat time makes you faster, not super magic motors.
It can be a great deal of fun and a great bonding experience. But only if the kid wants to do it......
Feel free to email me with questions.
-Rob
"I berkleyed up the kink!" My 8 year old after losing a race by .025 seconds.