Is it normal to have stretch marks at 13?

If you gain weight : Rapid weight gain can occur at any age. Hormones can also set this off. The layer of skin called the dermis looses elastin and collagen when there is a rapid accumulation of fat in the breasts, upper arms , thighs, abdomen, due to hormones. Weight gain, pregnancy, or other stressors on the skin where you accumulate fat. The dermis tears and it causes discoloration to the skin.