As parents, it is natural that you will fear for your child’s safety when they are out of the home, no matter how old they are.
However, it is best to give your children some independence so they can get some life experience and also enjoy with their friends. The time frame should depend on their age, as well as the conditions of the city and the neighbourhood.
The transport and security factors are also important to consider - will they come back alone or can you have a car arranged? How will they communicate with you when they are outside? Set rules and schedules keeping in mind all this information.