It really depends on your bone structure and gender. Prepubescent teens are also typically on the lighter side. Most will round out during puberty and throughout the teen years if they are eating healthfully.
You may want to talk with your parents/guardians about it. A trip to the doctor will help determine your ideal size for your frame, height and gender. If you are determined to be underweight a referral to a nutritionist will most likely be given and/or possible medical causes will be ruled out.
I have a sister-in-law who eats like a horse and is about the same size as you are (she's 20 and has been a "twig" her whole life
). She weight-lifts, jogs, does tae kwon do and she just turns into a firmer twig
. Yet her sister is the same height and weighs 160 (they do their weight training, jogging and tae kwon do together) and doesn't look overweight or unhealthy. Both are beautiful healthy young gals.
As for males my husband can also eat enormous amounts of food but when we were first married at 5' 10" he weighed only 130lbs. (20 yrs. old). That was 10 years ago and he now weighs around 160 (he weight lifts and still looks very lean). His older brother is considerably shorter and weighs...ahem...considerably more
yet he eats less.
So anyway, don't stress too much. Focus on eating a healthful diet, get a check-up and avoid being sedentary.
Love and Prayers, Kelly