Young teen bodies become intoxicated faster and with less alcohol than adults. Alcohol can alter vision and judgement and slow reaction time, so critical to safe driving.
Teens are inexperienced drivers.
They don't have as much practice in reacting to emergencies. Alcohol is especially dangerous when combined with this inexperience. Fatigue, which compounds the effects of alcohol. His friends let him drive, even after he scraped the guard rail. Obviously, he could have been killed, but there was another moment, when he fell asleep the first time.