teenager ... should i gain or not?

Hi I am 15 years old. I am 5'6" and I weigh 104 pounds. Do I need to gain? Is this an ok weight for someone at my age. I tend to eat about 1300 calories a day

i have a feeling you asked because you suspect that you need to gain, and you're right because you're underweight and should be consuming a minimum of 1500 calories, why are you eating so little at such a low weight? honestly, you should allow yourself to eat like a normal 15 year old, meaning not counting calories and obsessing over your weight. it is really a terrible obsession that can spiral out of control. it's great that you asked and I hope that you will consider eating more and getting your weight up!!

you're barely eating and underweight.  why are you eating so little?

yes, you need to gain, and should be getting almost twice that in food.

How long has your weight been that low?  How long have you been eating that little?  Sweetie, you have a  BMI of 16.8 and 19 is considered underweight.  You need to weigh at least 125 to get your BMI back in a healthy range.  You are a sick, young woman who needs to see a doctor ASAP to see if you've developed other health problems by being so underweight.  Tell your mom and dad and have them take you to the doctor.  Hopefully the doctor can recommend a dietician or specialist to help you.  In the meantime, try to up your calorie intake by 200-300 calories/day until you're in the 2000 range. 

YOU CAN DO IT and everyone here will help you. 

Just posting to say that I have very similar stats to you. I'm 15 and 5'6" and was 100 at my lowest, and 103ish when I decided to gain weight. Now I'm around 107 and feel so much better already. I've even grown a quarter of an inch (not saying it's due to eating more, but who knows?)! You definetly should be eating more, seeing as you're probably still growing. At least 2000 calories, 2500+ if you want to gain (which I think is a good idea). Good luck to you! =)