6th Graders

Something I've been pondering:

Here in Texas, 6th grade is most commonly a part of middle school. In other states that is sometimes not the case. The 7th and 8th grade kids here always say that, in general, the 6th graders are "so immature" and "more like 5th graders." I would agree that they are much more like elementary school students than they are like their older schoolmates.

So. When 6th graders are included in elementary school, are they "immature" as 7th graders, where perhaps that first year of middle school is a year of social development, with a larger school, more independence, more responsibility, etc.? If so, it makes that first year of middle school a valuable time for growth. Or are 12-year-olds just hitting an age where they are naturally maturing physically and emotionally, where the grade in school is of minor importance? If there is a true jump in maturity there, maybe 6th grade belongs in the elementary schools.