If the bike is for a child, they generally fall into age categories based on the average size of a child of a specific age. There is no hard and fast rule that says that you must buy a bike of a particular size for a child of a specific age. If the child is comfortable and not too stretched on the bike, and they can manage all the controls ie turn the handlebars safely, reach the brake levers and operate them AND touch the floor with the ball of their foot, then there is nothing to prevent you from selecting a bike in a bigger age category.

First Bikes; 12" & 14" wheels, suitable for average ages 2½ up to 5. Larger and slightly older children should benefit from the 14" wheel bicycles.

Once a child has learnt to ride without stabilisers (generally about age 5) they will move up to a 16" wheel cycle. These do not normally come fitted with stabilisers but they can be added to virtually all 16" wheel bikes if required.

Once you get into the realms of 20" wheel bikes and above, you may be offered multiple frame sizes. This has the benefit of being able to fit a younger child onto a bigger wheeled bike if they are confident riders. Although you will find that size options are generally not offered on a particular model, rather that different models have different frame sizes.

BMX bike sizing chart