In general, yes, but certain restrictions apply. One important factor that varies from provider to provider is the minimum age. With FlixBus for example, children are not allowed to travel alone until they are 10 years old. In most cases, a declaration of consent from the parents is required. Many providers require this in written form, while FlixBus, for example, offers an online form. Please note that a person must often be specified to pick up the child at the destination. This person must be able to identify themselves to the bus driver. For international connections, the entry regulations of the respective countries also apply.