Can you be classed as depressed if you under 16?

Yes. Depression has no age limitations. See your primary care physician who may run a few blood tests (thyroid and others) and may refer you to see a psychiatrist and/or a psychologist. If this is depression, a combination of medication and psychotherapy leads to good results.
Absolutely! Depression is not at all dependent on age and it definitely occurs in adolescents and children. The diagnosis usually rests on someone having >2 weeks of several of the following symptoms: depressed (or irritable) mood, feeling less interested in life, problems sleeping, changes in appetite, poor concentration, suicidal thoughts and sometimes others.