18 seems a bit young for someone to start growing any hair on the sides. For me, I get a few random hairs that grow on the sides (mostly towards the back near the ears. but they are not visible very well, thus I never felt the need to shave them. I just shave them off when I ever I shave off the mustache and goatee), but sideburns and the like hasn't even started to grow yet and perhaps for me never will.
I'm 26. At your age, not even the few small random hairs grew yet on the sides. So it may be too early to tell for you maybe?
<----- The kind of facial hair I grow is shown pretty well in my avatar by the way.
Though it hasn't grown longer yet in that photo, I shaved a week or two back and it hasn't fully come back yet in that photo.
The goatee part of my facial hair stops right around near or slightly past the edges of my mouth and lines up with my mustache. It grows past the edges just enough to wrap around and connect to my mustache.
As for how far down it goes towards my neck, it thins out half way past the chin on the underside and the final few hairs grow half an inch or so above my adams apple. As far as I can tell I consider my facial hair rather normal. It has a slight reddish shine to it in certain light conditions. I had red hair when I was first born since I have a little Irish genes from my Father's side of the family but turned brown in color later on though. Now the only hint of it is in my facial hair.
Generally, I let it grow for a little while, but I eventually shave it all off as facial hair is not very comfortable for me in a long term. I constantly fiddle with it and sometimes zits like to hide them selves in it.
So once it gets too long, I shave it all off in one go. I have yet to trim it or do anything fancy with it.