AvailabilityAvailability is one characteristic tweens look for when choosing friends. True friends are people they can see frequently, who are ready to do activities together and who are not always busy with other friends and commitments. Practically speaking, this means that tweens' friends usually go to the same school as them and/or live nearby. Therefore, if a good friend of your child moves or transfers schools, don't worry when the friendship dies off. This isn't an indication that your child has poor social skills; rather it's a sign of the normal developmental need for friends who are frequently present.
SimilaritySimilarity is important to both young children and tweens, but in different ways. Young children typically look for friends who like similar activities as themselves. Tweens, however, look for friends who have similar personal characteristics to themselves, such as attitudes, beliefs and goals. Being the same age also becomes more important than it was earlier in development. Tweens perceive a meaningful difference between themselves and people in the next grade down. In other words, your child and your friend's slightly-younger child may have been best buddies in toddlerhood and early childhood, but those days are probably over. In adulthood, though, exact age will become less important yet again.
Same GenderIn an extension of similarity, gender also tends to be an important factor in tween friendship choice. Children begin to prefer to play with peers of their same gender as young as 3 years of age, but don't become obsessed with gender separation until about 6 years. During the tween years, desire to stay away from the opposite gender (mixed with a simultaneous curiosity to know about people of said gender!) is usually in full swing. While there are of course notable exceptions to this rule, tweens' true friends tend to be of the same gender as themselves. This all changes, however, with the onset of puberty.
Actually Being Worthy of the "Best" in "Best Friend"During the early tween years, your child may be racking up "best friends" like it's going out of style. This is normal; each year until age 11, children tend to accumulate more and more people they call "best friends." Around 11, however, tweens usually become pickier about who they call a "best friend." They start to realize that "best" should denote something important. So don't be concerned if your older tween has fewer "best friends" than he used to - the friendships are probably the same, it's just the label that has changed.
A Respectable CharacterFinally, tweens report that they want friends who have positive characteristics, including generosity, loyalty and commitment. In other words, they only consider people they respect to be true friends and they steer away from the mean girls and guys. In contrast, younger children don't report paying attention to character when choosing friends. Interestingly, one character feature that tweens don't yet look for in a friend is the ability to keep a secret. That friendship requirement emerges during the teen years when keeping things private becomes an urgent matter.
Cook, Joan L., & Cook, G. Child Development: Principles and Perspectives 2005. New York: Pearson.
Gurian, Anita, & Pope, Alice. Do Kids Need Friends? Connect for Kids, Youth Policy Action Center.