Boys and girls undergo a number of physical changes during puberty, most notably the development of secondary sexual characteristics. These changes don't all happen at once. As is common knowledge, girls generally go through puberty earlier than boys. Girls usually enter puberty somewhere between 9 and 13 years old, while boys tend to enter puberty sometime between 10 and 14 years old.
The average age for girls to begin to develop breasts is 11 years old. Breast development is normally a girl's first indicator of puberty. In some cases, pubic hair may begin to grow first, but this is rare. Even with the breasts developing first, pubic hair growth often begins shortly thereafter, around a few months later. A girl's breasts will reach nearly full growth around two years after they've begun to develop and reach their final size a few years after that; meaning, the average girl will have fully developed breasts by the time she is 15 years old. Menstruation will usually begin roughly two years after the onset of puberty, which means around 13 years of age for most girls.
The earliest sign of puberty for boys is usually testicular growth, although this will normally occur so closely to the first growth of pubic hair, you'll probably notice the pubic hair first. The average age for boys to begin puberty is 12 years old. The voice will begin to crack around a year later, developing into the full, mature voice in two more years. Armpit hair will follow around two years into puberty, with facial hair coming around four years into puberty. The wet dreams should start a year or two into puberty.
If a child enters puberty early, it's called precocious puberty. If a child shows no signs of puberty by age 13 for girls or 14 for boys, it's considered delayed puberty. Regardless of when a child's puberty actually begins, the timing and sequence of the developments will likely proceed in the same manner.