Buying a bike for a young rider can be harder than it first appears. Many small bikes are not truly designed around young bodies; they are often scaled-down versions of adult bikes. That can mean cranks that are too long, handlebars that are too wide, brake levers that are too far away, and stock gears that are far too big.
The aim is not to buy the most expensive bike, the lightest bike, or the bike that looks most like a pro race bike. The aim is to find something the rider can control, maintain, grow with, and enjoy.

PPV has created this helpful guide to assist with buying a bike :

PPV Guide – How to buy a bike