First of all, I want to say that having a defined age where one becomes an adult (and thus all the things legally an adult can do) is a good thing, even though it is an arbitrary and artificial definition. I will get back to this point shortly.
All the posters that are attacking Holly and her post, you would make yourself look a lot less like fools and zealots if you can put forth your points logically and keep this as an intellectual debate. This is clearly the intention of the original post. Chilli, I think you are the one that hasn't thought through Holly's post before you attacked her. She said there are some interesting points in the article (which she has subsequently explained), she did not say she agreed with everything that was discussed in that story. You picked out the obvious flaw in that story and then attacked Holly based on that, even though Holly did not say she is in agreement with that point, a obvious straw man attack.
Definition of adulthood is a tricky thing, not only due to difference between society, but of time (people used to have a lot shorter life span, and thus have to "grow up" a lot quicker), and of course individual differences. The societal differences are what led to difference in definition of adulthood around the world, and difference between individuals is precisely why an arbitrary (but consistent within a given society) definition of adulthood is necessary. It is a comprimise, but a necessary one to protect the young.
Personally, I am one of those people that feel they are born old. I felt mature beyond my years when I was young, and if anything, getting younger as I grow older. I can definitely say though, I didn't not feel any different a month before my 18th birthday, and a month after. The only difference is by law I have become an adult.
Even in the area of law, the definition of adulthood is not cast in stone. Often we hear some teenager get tried as an adult because their crime is serious. Just go to show, nothing is in black and white. Having a consistent definition based on age is a good thing, and the ability to have exceptions based on exceptional circumstances is also a good thing (such as serious crime committed by a 17 year old that has repeatedly commit serious crimes).