Although on the average, girls do go through puberty before boys, there's a lot of variation regarding when puberty occurs in each youngster. In girls, puberty starts around the ages of 8 to 13 years, while in boys, puberty starts between the ages of nine and 14 years. So, if you have a girl who starts puberty at the age of 12 while her next door neighbor, a boy, starts at 11, there you have an example of a boy who started puberty before a girl. It can also happen that the girl is a slow developer, so she won't finished with puberty until she's 17, while that next door neighbor already finished by the age of 15.
A related question is whether the physical changes in puberty always follow a set pattern, and again, the answer is that people are different and while some teenagers might start puberty with acne and underarm hair, others might start puberty with an increase in height and pubic hair. Just as we don't expect all babies to sleep through the night or crawl by a certain, exact age, we don't expect all teenagers to follow a strict schedule of puberty as if they're commuter trains that leave the station at 8:23 am precisely.
Early puberty (precocious puberty), when girls start developing at the age of seven or when boys start at eight, is sometimes a normal variation of puberty, or it could be the result of a medical condition that should be corrected. Delayed puberty could be caused by nutritional deficiencies or lack of certain hormones. Early and delayed puberty often run in families, but tests (blood tests to check hormones, X-ray of the hand to determine if bone growth is normal, and/or chromosome studies) are available to determine if there are other reasons for early or delayed puberty.