Reading these posts has been very hard - because I put my parents through the exact same shite your children are doing now.
I came from a 'decent' background - loving parents, financially stable, good school. Up until thirteen I was very academic, consistently top of the class and they had high hopes for uni etc.
At around age thirteen, I started using drugs (mostly cannabis and cocaine), skipping school and getting into trouble with the police. I didn't live at home full time from about fourteen upwards. My parents couldn't keep me there - they couldn't tie me up in my bedroom and I spent the next couple of years sofa surfing at friends house, in squats and even sometimes sleeping rough.
If I was feeling 'generous' ie I had run out of people that would put up with me I would deign to return to the family home. Nothing moved me - my fathers anger, my mothers tears - I just didn't care. After all, what did they know about life?
I officially left home on my sixteenth birthday when I moved into a homeless hostel - not my parents choice, btw. I was sexually exploited and abused both physically and emotionally by the people around me - all of whom were much older and many of whom had mental health and drug and alcohol issues.
Social services had been involved for some time and in had a lot of counselling and support workers. It didn't help - there was no crystal clear reason for my behaviour apart from I was convinced I was right and the rest of the world was wrong.
Even now, as an adult with the benefit of hindsight I don't know what was behind my behaviour.
I'm in my late twenties now. I am married to a wonderful man who respects me, own my own house and have a good job as a chartered accountant. There was a turning point where I thought 'woah, enough is enough' and thank god when I got there, my parents were still there to support me in building my life - and thank god I didn't get to the point of no return.
My parents tried everything to help me when I was younger - the counselling, tough love, disengaging....but if someone is determined to go there own way, they will do. Sometimes things just happen, there isn't a reason why, or anyone to blame, I think it's just the intrinsic nature of the individual.
Anyway, sorry for rambling on. I'm sure you all feel very helpless and that you are bystanders watching your beloved offspring destroy their lives. I just wanted to say, always leave the door open for them - not neccessarily to come back home or as a 'get out of jail free card', but emotionally and mentally, if you can.