Once children discover cooperative play, they increasingly seek out peers to share in the fun of playing. As young as three, children begin to understand that in order to keep a playmate around, you must take turns, share, and compromise.
Around ages six to nine, the concept of reciprocity and mutuality is present. Friends are chosen based on mutual interests. They develop increasing awareness that their classmates have different viewpoints from their own, and with each year find this to be a valuable contributor to their friendships, as it adds novelty and enrichment. Children also become better at interpreting the emotions and intentions of others and taking the perspective of their friends. They engage in increasing prosocial acts of sharing and helping their peers.
In preteen years, from ages ten to thirteen, friendships involve an increasing exchange of ideas, asking for opinions, and acknowledgment of one another’s contribution to the conversation, play, or activity. Friendships at this age are based on common interests and how each can help the other. Children are more careful about what they say and do, knowing it might hurt someone. Friendships last longer at this age and more value is placed on personal attributes of loyalty, trust, and keeping promises.