AD in friendship

What is the real big deal in this life – is the friendship. Friends are people who stay next to us during our entire life. They are those we call when we feel bad and need a shoulder to cry on. They are the very first people we call when we are extremely happy and want to share that happiness with someone. Friends are people who realize our most secret thoughts even if we are silent about those.
In most cases people find friends in their schools, universities, work and so on and so forth. So, in most cases friends are people of the same age. But, what is that rule if there are no exceptions of it. There are exceptions of all the rules and so there are exceptions of the rule about friends are people of the same age. Under some unbelievable conditions people of 6 or 7 or more age disparity become friends.
It is not a big deal if both of them are adults and there is no big difference between their lives and believes. But, what the age disparity is in case if friends are people of different ages and one of them is fifteen and another one is twenty-three? Are there are any common interests for those people? Are there are any common stuff they have in order to have fun together or else?
I bet there are some common interests and stuff, as there is no way for people to be friends otherwise. The elder friend is like a modified model of parents. He knows more about this life than his or her younger friend, and he or she knows the better way of explaining those things to his or her younger friend. The elder friend is wise enough to give reasonable pieces of advice and young enough to make another listen.