While it is possible to buy training wheels for a 20-inch bicycle, doing so is usually not a good idea, according to bicycling expert Sheldon Brown. Training wheels should only be used for the first two to three years of learning to ride a bike at most.
Training wheels more typically come included with 12-inch or 14-inch bikes, which are intended for use by children who are 2 to 6 years old. Twenty-inch bikes are used by children aged 7 to 10, who should already be able to ride without training wheels. A child with a developmental or physical disability may still need training wheels when riding a 20-inch bike, but specialised bikes may be more appropriate.