When a person enters upon other people, does he greet the oldest  first or the youngest?

This is an issue which some people do not understand. When
you meet someone or you enter upon someone and you want to greet them (by shaking
hands) or by offering tea or coffee, then you should start with the oldest,
because the Prophet

(peace and blessings of Allaah be upon him) had a miswaak (tooth-stick) in his
hand and there were two men in front of him. The younger man wanted to take
it, but he said to him, “The older, the older.” 

This is different to when there is a young man
on one’s right and an old man on one’s left, and one is sitting
between them. In that case one should start on the right, even though that man
is younger. 

On this basis, when a person enters a gathering, he should start with the oldest, but if he wants to give something, then he should start with whoever is on his right, not on the right of the oldest.