My thoughts have changed as I've gotten older. 18 seems like such an arbitrary age to call someone an adult.
Scientifically, we may still be growing past 18. Our brains aren't fully developed yet.
In our society, there are weird laws and practices for the ages 18-21 as well. Some things you can't do until you're 21. Some things you won't be penalized until you're 21. This really goes on until 25 for some items, but it is understandable because some haven't reached the appropriate maturity.
As our life expectancy has gone up, and college has become the norm, it seems more and more that 18-22 are still kids to many.
If we call them adults, they should get full privileges of being adults. Most people I know usually refer to people in college as "college kids". Like, oh yeah bobby, nice college kid.