L&S: Adulthood, Then and Now

James Taylor is working his way through J. S. Mill's "Harm Principle." There was a brief exchange with a student about how Mill defined an "adult" for the purposes of the "rational adult" in his theory. James' response was that, in fact, "rationality" defines adulthood. We know an adult because he or she is rational. Ignoring the complexities of determining rationality, I'm more interested what this means in a society where adolescence has been extended in the ways that it has been in our own. Would Mill's rationality test have likely included more children of younger ages than would be the case today? Would a 13 year-old chosen at random been more likely to meet the rationality test in the mid-19th century than today?