It is difficult to successfully train many children to walk before they are 10 months old. this best illustrates the importance of

This best illustrates the importance of maturation.

Maturation refers to growth, both in physical and psychological sense of the word. Children usually don't start walking before they turn 10 months because they are not mature enough yet to do so - their bodies still cannot support their weight, and they might not feel ready for such a daunting task.