Hello!
First off, I thank you for a cool thread to write in. Here's my opinion.
I believe that 13 years is not near enough time for an individual to develop. Sure...there are exceptions, but these exceptions are extremely rare. Most 13 years olds have not lived enough...simply...just not enough years under their belt.
I remember when I was 13, I thought I had seen everything...and that fact is simply not true. There is a logical reason why adulthood is established at 18. It is about the average age when people are capaple of compiling all their previous experiences in life, and making a logical decision from these experiences.
It's very hard to believe...but now that I'm 22, I definitely realized how childish I was at 13...but really, there are more mature 13 year olds than myself...but...that is rare...and therefore...it is not considered adulthood.