If you want to specialize in just adolescents, then you really need to complete a child/adolescent fellowship. Once you finish training as a C/A psychiatrist, you have tons of opportunities in either private practice, group practice, or academics. Most C/A psychiatrist see a full range of ages from young children to adults. However, you could easily limit your practice to just children and adolescents. Regarding only seeing adolescents, I would imagine it wouldn't be a problem to limit your practice and put yourself out there as an "adolescent specialist." There is a high demand for that age and it should be doable in private practice. If you don't mind seeing slightly older adolescents and young adults, then working in a University mental health clinic is a great option.
I would contact some of the C/A psychiatrists at your medical school to get more input on this. I'm still a fellow, but that is just my two cents...