Do you think your DC migrates towards other kids of the same age (in their preschool class)?
My DD goes to 4 year old preschool, and they also spend about half their time with 3 year olds. I just noticed lately that she seems to be attracted to the kids of the same age. The only reason I even noticed was we got a list of the kids in the class (for Valentine's day) and it happened to have all their birthdays, and my DD's best friends are all turning 5 in March/April (they are the oldest in the class).
We even do playdates with a variety of ages of kids - she plays well with all ages- but she definitely seems to click well with the most similar age. Not that this is any kind of epiphany, but I just noticed how those consistently close kids are the same age within a couple months, but maybe mostly since they are the oldest in the class as well.
Re: Migrating towards similar age in class