Everyone is different. We all have different shaped bodies, eye colour and our weight varies from one person to an other. It is not healthy to be too heavy or too light, but if you have a look at this table you should be able to see where you are. First check your weight, then look at what most people weigh. If you are between the red and blue lines you are in the normal range. Pink is average.

If you are not, don't worry, you may be tall or short for your age which can affect this, but it may be worth talking to a doctor or nurse about it.