Children are allowed to travel alone when they reach the age of 12. If necessary, airport and airline employees will ensure that the children’s travels are smooth and stress-free. You should, nevertheless, check your airline’s policy before travelling because there could always be exceptions (e.g. Ryanair aircraft have an age limit of 16). It is also recommended that you always have your child carry documented parental consent that the child is allowed to travel alone.
Additionally, most airlines have an Unaccompanied Minor service where the airline takes responsibility for a child aged 5-11 getting on board the aircraft and being delivered to the destination. If you wish to use this service, you should notify the airline well in advance. In most countries, written parental consent containing information on where and when the child will travel and the necessary contact details is sufficient to have a child travel alone. There are, however, countries where notarised parental consent is required. When in doubt, you should review the immigration requirements of the respective destination country.