Should i be worrying if i'm only 18 and really skinny?

It depends: It depends on how skinny you are, and why you are so skinny. If you simply have a fast metabolism and can't eat enough to keep up, you are probably fine as long as you are fit and eating a good, healthy balanced diet. On the other hand, your skinniness could be caused by a metabolic disorder or nutritional problem. You should seek medical attention to find out which applies to you.

May be not: If you are healthy and skinny you probably should not be worried. But always get checked with your doctor, so that we are not missing anything.