I agree with this completely. I'm only 19, I had an EMT job at 18. I have a lot to learn about medicine and even life in general, but that doesn't mean I can't handle things in a mature manner.
I'm not blowing my own horn, just describing myself, but I've always felt a little bit separated from my age group. Most of my friends are a good 2-5 years older than me. Doesn't seem like much, but when your buddy's buying his first beer when you're taking your driver license test, it seems like quite a bit.
On top of that, I graduated in my junior year so I suppose I just never wanted to deal with the high school shenanigans.

h34r:
Do I have my moments where I "act my age?" Of course. But not on duty. I take my job very seriously. When it's go time, I get going.
It is kind of frustrating that I can be responsible for the health and safety of my partner and the complete stranger on my gurney, but can't enjoy a beer with my burger at the Sunday BBQ.
However, I think 18 is a good age for someone to be providing routine patient care. At that age, one is legally responsible for the actions they take, and should bear that with due gravity. If they're not mature enough for the job, chances are they won't last long before they're booted out of the field. Though I suppose that's not always true. One can hope, right?