Whether they appear during puberty or pregnancy, stretch marks are an embarrassing side effect of rapid skin lengthening. Rapid and extensive stretching of the skin breaks down the skin's collagen and elastin fibers faster than they can reproduce or elongate. These fibers expand until they snap, leaving the top layer of the skin sitting on an uneven and fragmented support structure. This is what causes the uneven, pigmented appearance.
Stretch marks are considered to be partially caused by hereditary, which explains why some people are more prone to developing them. No matter how you got them or how long since they appeared on your skin, there are ways to make them appear much smoother and more even-toned.
Some products to consider are Mustela Stretch Marks Intensive Action, StriVectin SD Advanced Intensive Concentrate for Wrinkles & Stretch Marks and Apothederm Stretch Mark Cream.