Approximately 4 out of 100 teenagers get seriously depressed each year. Sure, everybody feels sad or blue now and then. You feel sad or cry a lot and it doesn't go away.
Life seems meaningless or like nothing good is ever going to happen again.
You have a negative attitude a lot of the time, or it seems like you have no feelings. It's hard to make up your mind. You forget lots of things, and it's hard to concentrate. You get irritated often. Or you wake up really early most mornings and can't get back to sleep.