When should a young child wear a helmet?

A young child should always wear a helmet when riding a bike or motorcycle. In most places, there are laws requiring anyone under a certain age (often 16 or 18) to wear a helmet.

Of particular note:

If the child is doing tricks on a bike, they definitely need a helmet.

If the child is unstable, they definitely need a helmet.

If the child is going at high speeds, they definitely need a helmet.

If the bike is in poor condition or has brakes that are difficult to use, they definitely need a helmet.

If the child is inexperienced with using the front brake, but does so anyway, they definitely need a helmet.

If the bike is too small and therefore too top heavy, the child definitely needs a helmet.

If the child has trouble following commands but still needs supervision, they definitely need a helmet.

In general, whenever there is a high risk of the child running their head into something at high speed, and/or with the force of their body behind it, they should wear a helmet.

Bikes move at high speeds. Even a child can reach speeds sufficient to cause severe injury or possibly even death on level ground, and anyone going down a hill can get sufficient momentum that even a helmet may not protect them from the worst of accidents. A 6% grade is steep enough that, given sufficient time, a biker will reach speeds in excess of 40 MPH, equivalent to the speed someone would be falling after they had fallen 60 feet. A head-on collision at this speed without a helmet would be like biking out of a 6th story window.