Watching your friends develop faster than you can be difficult, even frustrating -- but it's rarely a medical problem. Your body will develop in its own good time.
If you're a girl, the typical range of ages for puberty to begin is nine to thirteen. Since boys start to mature later than girls, the typical age range is between nine and 15 years old for you guys. If you're within these ages and puberty hasn't started yet for you, you're just going at your own pace. Ask your parents when they started puberty; it's very possible one of them was a late bloomer too. If you aren't eating properly, you might be delaying your puberty. For the girls -- if you're very athletic, this can also lead to late development as your body fat percentage may be too low.
You might be experiencing delayed puberty if you are 14 years old and haven't begun to show any of the typical physical changes of puberty. If this describes your situation, do ask your parents to take you to a doctor so he/she can determine the cause and get you started. There are a few different reasons your puberty can be delayed. Sometimes, if you have a disease, such as cystic fibrosis, asthma or kidney disease, you might experience delayed puberty because your illness slows your body's growth. The better you can manage this kind of disease, the more likely it will be that you can bring on puberty.
In other cases, delayed puberty is caused by chromosomes that aren't working properly. Girls are supposed to have two X chromosomes, but if one is missing or damaged, this leads to a condition called Turner syndrome. Boys should have one X and one Y chromosome, but sometimes a boy will have an extra X chromosome. When this happens, the condition is called Klinefelter syndrome. Both of these syndromes can delay puberty. Don't worry - both conditions are treatable and your doctor has options to help trigger puberty in case of either syndrome.