There is a rapid formation of bone mass in the fetus and infant. This slows somewhat during childhood until age 11 in females and a year or so later in boys. During the growth spurt which which accompanies adolescence, tremendous bone formation occurs. The vast majority of adult levels of bone mass are achieved by age 18 or so, with only a small amount added until about 28 years old.
There are a number hormones which are important to this rapid formation of bone during the first two decades of life. These factors include, estrogens in females, testosterone in males, and growth hormone, and some other "minor" hormones in all persons.