Adrenarche actually refers to the maturation of the zona reticularis in the adrenal glands that occurs during puberty, leading to the increase in the expression of sex hormones. It is reflected in the secondary sexual characteristics as the appearance of pubic hair.
Premature adrenarche is defined as the appearance of pubic hair before the age of 8 in girls and before the age of 9 in boys.
See also thelarche and menarche.