Part of the problem is having only 2 "modes" of existence -- child and adult.

For purposes of trial for crimes, legality, etc. you're either a "child" or "adult", which magically changes when you attain the age of 18 or 21. That's obviously ridiculous, as change is much more gradual than that.

In our legal system, a man who kills someone the day before his 18th birthday will get a far lighter sentence than a man who kills someone ON his 18th birthday -- assuming the crime committed was exactly the same. Of course, the prosecuting attorney would likely move to "try him as an adult" in the first case, but still -- by default, the age of 18 is considered magical.

The same is true about the age of reason. A child doesn't go to bed the day before his 7th birthday as dumb as a baby and wake up the next morning as rational as an adult.