I could write a treatise on this! I've been through 3 boys that at one point or another turned 18. My oldest was easy as pie, bright, motivated and generally agreeable. The worst he did was drink a few beers on the weekends with his buddies his final year of high school. I knew it. I looked the other way.
My middle son was another story, headstrong from day 1. Brilliant, but cocky and seemingly hell bent on learing things the hard way. He also ahd a drink problem. I sent him to rehab. I let him know all amenities, phone, food, shelter, after 18 were at my pleasure, so it was follow the rules or go get your own place. He straightened up.
My youngest, well, he was another story. Truth be told, I had to let him figure his shit out. He is also extremely bright. There was no reason for him not to finish high school. But he wanted to party. The school even offered to allow him to come, sit in a room for 1 hour a day to do the work and they'd give him the damn diploma and he couldn't bother. I put him out. Not becasue of the school thing, but because he kept bringing party pals over after my repeated requests not to. He hated me for a while. Then, he got his ass back to school when he found out that life can be mighty tough when mommy isn't there paying for everything and cooking meals. He's 27 and working in a good paying job.
With my youngest, I think there were a few issues, one of which was the actual high school system. As bright as he is, his mind isn't geared toward that type of learning. He's a hands on guy.
It's so hard, we want the best for our kids. And at 18, they think they're grown. We did too, but it was a different time.