They occur when the skin is pulled by quick growth or stretching. Usually the skin is quite elastic, but if it is overstretched scars and stretch marks form. When they first form they tend to fade over time.
You can use creams and foundations to make them less obvious, but if you want to get rid of them altogether you will have to see a dermatologist or even a plastic surgeon.
The options available range from surgery to laser treatment. - kidshealth.org;.bodyrenewal.co.za; medicinenet.com