What is the ruling on a boy accompanying a girl? And until what age?

If the children are below the age of discernment, there is no harm in some of them accompanying others. However, if they have reached the age of discernment, they must be prevented from this, as a way of تربية (educating) them in virtue, preventing the causes of vice, and blocking the pretexts of fitna (temptation/strife). Those who have reached the age of accountability (تكليف) or are close to it must also be prevented from this.