Hi,i noticed my daughter who is 14 years old,lost hair behind the ear.what could be the cause and will they grow back,if yes how long would it take?

She may have a condition called alopecia areata and this is often due to an autoimmune problem. Also low thyroid conditions can cause loss of hair. She should see a doctor or an endocrinologist for follow-up.

It is hard to be sure without an evaluation. One of the more common reasons at that age is hair twirling/tugging as a habit. Called "traction alopecia" it is hair pulled out whether conscious or not. Nervous habits can be the source or a medical condition. A history/exam and maybe testing are appropriate.