Most american children develop traditional gender-role beliefs, such as that girls "become nurses," by what age

The answer is at the early age of four years old because a child at four years of age has the idea of gender roles in which he or she knows what his or her gender is and what he or she should do as a girl or a buy that made her or him come up with an idea of what she or he would likely want to do when she or he grows up.