They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

 We do not say that there is no thought of age groups in this. There almost certainly is; but John has a way of saying things which can be taken in two ways, a narrower and a wider; and, while the narrower meaning is here, we must go beyond it to find the full meaning. 