If my depression/anxiety started in childhood/teens for no apparent reason (no history of abuse/neglect/trauma), does this mean it's caused by genes?

Hereditary component: Science does suggest some genetic component to psychiatric disorders as we see clustering of cases in different generations of families with mental illness. The exact cause and patterns if transmission has not been determined. But other factors, besides h/o abuse, trauama and neglect, can cause depression. Pl get evaluated by a qualified mental health specialist soon.

Could be: Depression that young could indicate some sort of biochemical imbalance. Could also be related to genetics.

Depression: Depression/Anxiety can be caused by a wide variety of issues. Everything from nutritional deficiency/imbalance, hormonal imbalance, heart conditions, etc can contribute to feelings of anxiety or depression. It is important to be evaluated by a physician and address any other health concerns you may be going through as well. FYI exercise is proven to work best to improve depression.