DS 9yo had a best friend since they were 2. They have been in the same class in preschool and elementary school more often than not. They took tennis lessons together and for a while, played on the same soccer team. We also go to the same church. They saw each other almost every day of the week.
Last year, in 2nd grade, DS got really frustrated with best friend because DS wanted to eat lunch with other kids, play with other kids at recess, etc. Best friend would get upset if DS did anything with anybody else. We dropped the tennis lesson, and asked the teacher to put them in different classrooms for 3rd grade but their friendship still isn't as strong as it used to be because best friend is still upset when DS hangs out with other kids.
I have tried somewhat to discourage myother DS 7yo from having one best friend. He has several good buddies but no one best friend. He's happy, so I'm happy.
At our school, cliques form as early as kindergarten. The boys don't seem as clique-oriented as the girls.