is it normal for a 12 year old to have hidden depression?

I'm so sorry, a lot of young people suffer from teen depression. Some causes may be physical or mental. But the bible tells us at Isaiah 33:24: "No resident will say, I am sick". There's a website that I often refer to to help with this question along with others you may need answers to, jw.org. 

ANONYMOUS.........THE SYMPTOMS OF TEEN DEPRESSION CAN APPEAR DURING ADOLESCENCE.

WHAT PARENTS CAN DO; 

• Recognize that depressed teens may find it hard to express their feelings or may not understand what is happening to them. They may not even be aware of the symptoms of depression.

• Teens tend to express their depression in ways different from those of adults, so be alert to major changes in your child’s behavior, eating habits, moods, sleep patterns, or social interactions—especially if the changes persist for weeks.

• Take seriously any comments or hints regarding suicide.

• If you suspect depression (not just the blues), consider getting a professional diagnosis.

• Help your teen adhere to the doctor’s prescribed treatment, and consult the doctor if you see no improvement or observe unpleasant side effects.

• Have a stable family routine for eating, exercising, and sleeping.

• Maintain good communication with your teen, and help him or her to deal with any stigma associated with depression.

• Because depression can make one feel alone, ashamed, or worthless, regularly reassure your teen of your love.

Depression anybody can suffer from it. There are always telltale signs and would be prudent the person in question not to be left alone.