Young. Since melatonin is a naturally occurring brain hormone, you can use it pretty young. The biggest problem is how to get it into a child too young to take it whole. Always go for the lowest dose possible (start with about a miligram). It is best used as a sleep cycle regulator; in other words, you use it to help promote a specific bedtime by giving it 90minutes beforehand. It's not a sedative.
Melatonin & children. I would not give it to any child until they are over 16 yrs. Old. You need to do behaviour modification on your child for better sleep hygiene and if it does not help see your pediatrician. Some children need to have a quiet time before bedtime wihtout any tv or other distractions. Sometimes even the night lihght keeps them awake. Talk to you md so they can guide you.