A 5-year-old who is between 24 and 36 inches tall generally needs a 12-inch bike, while a 5-year-old who is between 34 and 42 inches tall generally needs a 16-inch bike. The size of bike a child needs depends less on the child's age and more on his or her height.
While the frame of an adult's bike determines its size, children's bikes are measured using the outside diameter of a tire, so a 12-inch child's bike means the outside diameter of the tire is 12 inches. A 16-inch child's bike means the outside diameter of the tire is 16 inches. To check the fit, there should be between 1 and 2 inches between the bike's cross bar and the child's crotch when standing over it.