I think hormones dictate another story for middle school years. When I skipped in elementary school, girls usually didn't puberty until middle school and those hormones bring about all kinds of issues. Now girls are entering puberty sooner. (part of that is attributed to non-traditional family -- since being around a biological father inhibits early puberty in an offspring) though they are finding early puberty around the planet. So you have a kid skipping a couple of years in elementary school being thrown in with girls that are entering or in the "boy crazy" stage.

Talking about chronological and social ages really are varaiable around middle school.