I'm not sure the poll goes back far enough - before the 20th century, people were considered adult around 13 or 14 years old in many cultures (including American) - and they probably are still in some cultures.
In my opinion, the fact that we treat people as children until they are so old increases the probability that they will act like children. The fact that we see so many immature 18 year olds is not necessarily a sign that the age of adulthood is too low - it seems to me more like a sign that we've infantized generations of potential young adults and created a state that allows them to be children far too long. (Compulsory education and the raising of the age of adulthood have, in my mind, created the very need they were supposed to address).
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[Last edited by silver at 04-13-2006 11:22 PM]