- Issue Time
Size 12 bicycle
If you're buying a child her first bike, it will probably be a size 12. These bikes are appropriate for most children in the range of 2 to 4 years old. They have a 12-inch wheel and an inseam -- the distance from the ground to the crotch -- measuring 14 to 17 inches. Most will come with training wheels, as they are designed for beginner bike riders.
Size 14 bicycle
The next size up is a less commonly available size. Size 14 bikes are appropriate for kids in the range of 4 to 6 years of age. These bikes have 14-inch tires and inseams of 16 to 20 inches.
Size 16 bicycle
Size 16 bikes will fit kids from approximately age 5 to age 8. These bikes typically have have pneumatic tires and coaster brakes -- or back-pedal brakes -- but some may have hand brakes. They have a 16-inch wheel and an inseam of 18 to 22 inches.
Size 18 bicycle
Size 18 bikes are another uncommon size of bike. These bikes fit kids from around 6 to 9 years of age. They have an inseam ranging from 20 to 24 inches and 18-inch wheels.
Size 20 bicycle
Some size 20 bikes have the features of typical adult bikes, like hand brakes and gear systems. These bikes are for kids aged 7 to 10. They have 20-inch tires and inseams measuring 22 to 28 inches.
Size 24 bicycle
Children aged 9 and over are usually ready for the full features of an adult bike. Size 24 bikes offer these features on a bike with a 24-inch tire and an inseam of between 24 and 28 inches.
Beyond Child Sizes
Preteens and teens are usually too large for bikes in the child-size range. Adult-size bikes come in small, medium and large frames, ranging from 16 inches to 22 inches in height. Adult bikes should have an inch of space between the frame and the crotch when the rider is standing over the bike.