Most boys complete their growth by the time they are 16 years old, according to KidsHealth. However, some boys may continue to grow another inch or so in their later teen years, and muscles will continue to grow into adulthood. Growth during puberty happens over time, so it can be hard to know when it has finished. The first signs of puberty in boys usually appear around the age of 12 years, and most reach their adult height by the age of 16, but this can vary. Boys tend to grow most quickly between ages 12 and 15, and the growth spurt of boys is, on average, about 2 years later than that of girls. Puberty can last anywhere from 2 to 5 years, so it is not always easy to predict when it will end. In rare cases, some males may continue to grow into their early twenties, but growth charts show that the majority of males grow very little after the age of 18.