How many calories do i REALLY need?

I am 13 but reeeeally small for my age. I'm 4'9 and about 75-80 pounds. I'm not under or overweight, but I can't figure out how many calories I need to be eating. I do enough excersize (30-60 minutes/day), but nothing too extreme. I just walk or go for leisurely bike rides or hikes, or jumprope/mini trampoline. I entered my data and it said that I needed about 900-1100 to maintain my weight. However, everyone on yahoo says that no one should ever go below 1200. I normally don't count my calories, but I did yesterday to see what seemed right. I only ate like 1000, and I didn't feel hungry at all. I have a big appetite, and I eat when I'm hungry, which is quite often, but I get really full quickly. is this really bad for my metabolism to go this low? It doesn't really feel low for me, just normal. I'm not trying to lose weight or anything, but I want to stay healthy. Is the 1200 calorie requirement just for adults who are much taller and bigger than me? Can anyone on here help me?

to the person who said 1500-2000, thanks for the answer, but I really can't stomach that many. Today I counted again, but tried to eat more, and I'm at 1000 after dinner and I'm really full. Although I haven't been counting calories other than today and yesterday, my guess is that I usually only eat about 1000. It seems like the number my body likes, and it keeps me feeling full and satisfied. My question is, is this a problem to my health? I'm not dieting, it's just how much I eat. Do I really need to force myself up to 1200?

I'm kind of in the same boat, but I think you should eat up to 1200. Just eat foods with higher calories - you don't necessarily need to eat MORE food, just higher-caloried food.

1500 to 2000, just make sure it's a balance of fats, carbs, and protein, they are all important