<p>I think you should look at anthropologists' work. Meredith Small has a book called <em>KIDS</em> which addresses this question, at least in part. There is a biological and cultural interface involved - we are the only species which really has an "adolescence" per se. Most go from infancy to childhood to adulthood without the long period between which we have as humans. I know some of the earlier chapters of her book address this.</p>
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<p>Failing to situate your argument within an acknowledgment that cultural ideology will inform each culture's definition of 'adulthood' would make any argument/chronology suspect. Some sociologists are arguing today that we have two different ages within our own culture for the onset of "adulthood." That is - the poor, working-class 'kid' becomes an adult at or around 18. But the well-off kid becomes an adult upon graduation from college (or from graduate school, even), with adolescence extending until people are nearly 30 at times (that they are relying on their parents for assistance, not fully independent, and not making the decisions which 'adults' make -- again, also a culturally situated argument). </p>
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<p>In many ways, a fifteen year old in a different culture might well be more 'adult' than a 20 year old in our culture, as a result of how we negotiate adolescence. They are taught skills (and expected to exhibit them) at an earlier age; at the same time, they are not exposed to the same level of complexity of knowledge which we are exposing our children to here (iPhones, computers, electronic gadgets of all sorts).....</p>
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<p>An example of the cultural situatedness of this question would be a conversation my sister and I had with my father several years ago. We were both in our mid-20's and home from grad school for Christmas. Dad asked us to do the "little girls'" chores that day - we both asked why they couldn't do them, themselves. He told us they needed a break and we hadn't been helping at home while we were at school, so the little girls needed a vacation. We had a pretty in-depth conversation and it came down to the fact that, as far as he is concerned, you're not an adult until you marry. So, my sister who'd married at 20 wasn't expected to do chores at home; but we single girls in our mid-20's were not yet adults. </p>
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<p>I think some would argue that marriage shows adulthood (it used to in our culture); others would argue financial independence (this is more difficult to argue in light of the recent economic situation, but does evince some of the discomfort which people feel when, as parents, they have to move in with their own parents for awhile); others might tie it directly to sexual reproduction age; while others might specifically tie it to completing an initiation rite (graduating from high school, 18th birthday, graduating from college).....</p>
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<p>Definitely, get thee some anthropological research!
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