When it comes to the right age for riding a mini bike, that depends on the size of the engines in question, your experiences as a rider and local laws. Mini bikes with 40cc to 50cc engines are best for kids between 6-10, while private property and off-road areas both remain great places for safe fun. Engines are usually limited to less than 20 mph, the smaller engines thus offer newbie young riders a chance of honing their skills because they do not interfere with the precise twist-grip functions.
Mini bikes are also a common and popular ride for older children -- generally ages 10 to 14 -- with engines between 50cc and 100cc. These bikes go from 25 to 40 mph, but with power available while being a pedestrian. That does not mean that parental supervision and the right equipment i.e. helmets, gloves, pads etc are key to minimizing injuries. Wearing a helmet decreases the risk of head injury by up to 70% while age should never be taken as a guarantee either way when it comes to safety precautions.
Some jurisdictions have statutes that clarify our point of view; stating no rider under 16 can legally ride a mini bike on public streets. They may require a license, similar to those of mopeds or scooters. In several U.S. states, riders of such mini bikes may be required to obtain a driver's license or motorcycle permit in order to legally operate on public roads. In these situations, the age to ride on the street is tied definitively to legislative requirements -- so before you turn your child loose in traffic, it would be wise to brush up on local statutes.
They will show that with the proper education and environment, kids truly do possess a high potential for learning premier level skills early in life as former great motocross racers like Ricky Carmichael learned how to ride mini bikes before they hit their teen years. But remember that this did not just happen: professional training combined with practice and so on in a controlled environment got this right.
In the end, age appropriateness is very much a function of not only the rider but also how strong the bike is and what kind of environment it will be ridden in. The mini bike is a safe and enjoyable form of riding experience for kids, teens or grown ups but it depends on the degree of regulation being followed by the owner that will determine how safe and amusing ride they might possibly have.