"I’ve listened to someone as young as 14 and someone as old as 100 talk about their close friends, and [there are] three expectations of a close friend that I hear people describing and valuing across the entire life course,” says William Rawlins, the Stocker Professor of Interpersonal Communication at Ohio University. “Somebody to talk to, someone to depend on, and someone to enjoy. These expectations remain the same, but the circumstances under which they’re accomplished change.”

http://www.theatlantic.com/health/archive/2015/10/how-friendships-change-over-time-in-adulthood/411466/