Friendship is generally voluntary and lacks formal structure. As we age, expectations among friends don’t change much. Over time circumstances change. You are no longer a student. Your priorities have changed. You may not be in touch with your friend for a long time, but when you run over him/her after a long gap, enthusiasm returns, expectations return. You want to watch a film, sing a song. The past becomes alive in the presence of a friend. I am not sure what phase of life is best for forming the bonds of friendship. Perhaps any time is the right time. Perhaps at a young age self-disclosures are easier. Perhaps self-discovery is easier when among friends.