Depending on the school district, middle school is anywhere from 4th to 9th grades, though 6 - 8 is pretty typical. (However, not universal, and people where middle school is 5 - 8 or 7-9 or even just 7 and 8 may think that the model they're used to is standard.)
Elementary school starts in first grade. (Actually, for many students it starts in kindergarten or earlier, and in some states kindy is mandatory, but let's simplify.) School districts vary wildly on cut-off dates, with the youngest child in a first grade classroom on the first day of school being anywhere from 5 years 9 months to 6 years 9 months even if the school district does not allow children to be either "redshirted" in first and kindy, that is, kept back, or advanced a year.
So students entering middle school are probably in their double digits but not in their teens.
Also worth noting: That middle school period is called different things in different areas, including "junior high" and "intermediate school". Properly speaking these all are supposed to refer to different models of schooling, but in actual practice that largely goes out the window all the time, especially if you, like most people, don't know that there's supposed to be a distinction.
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Elementary school starts in first grade. (Actually, for many students it starts in kindergarten or earlier, and in some states kindy is mandatory, but let's simplify.) School districts vary wildly on cut-off dates, with the youngest child in a first grade classroom on the first day of school being anywhere from 5 years 9 months to 6 years 9 months even if the school district does not allow children to be either "redshirted" in first and kindy, that is, kept back, or advanced a year.
So students entering middle school are probably in their double digits but not in their teens.
Also worth noting: That middle school period is called different things in different areas, including "junior high" and "intermediate school". Properly speaking these all are supposed to refer to different models of schooling, but in actual practice that largely goes out the window all the time, especially if you, like most people, don't know that there's supposed to be a distinction.