Child, parent and grandparent For the first 15 to 20 years after birth, the human body grows in weight and height. This growth is controlled by hormones, which are chemical signallers controlled by the brain. Other changes take place in the human body as we go through life. Puberty or adolescence is when a child’s body experiences a series of physical changes as it matures into an adult’s body capable of sexual reproduction. Almost as soon as the body is mature, the process of ageing, or senescence, begins. The way in which individuals grow, develop and age is dependent on factors such as genes, diet, lifestyle and environment.
At birth, the average baby weighs about 3.5 kilograms (7 pounds 11 ounces) and is around 50 centimetres (20 inches) long. Humans grow fastest between the ages of 0 and 2, then at a slower but steady rate until puberty. A growth spurt starts usually at around 11–12 years old in girls and 13–14 in boys. The rate of growth then slows to zero, usually by about the age of 16 in girls and 18 in boys. The ages at which young people begin and end their growth spurt can vary widely between individuals.
The tallest nation is the Netherlands, where the average man is 6 ft (185 cm) tall. The shortest nation is Indonesia, where men are 158 cm (5 ft 2 inches) tall on average.
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