Since you are only 13 years old, you are still going through puberty, then this can be very normal. Your body is going thru many changes at this time and sometimes the skin is being pulled or stretched during your rapid growth period and the body developes fine lines. Most of these will show up on your breasts, hips, and backside. So I would not worry so much about them, they usually either go away or may just fade over time.
It depends, stretch marks come from two things: getting bigger, or getting smaller. I have stretch marks because I was overweight as a kid, and was really short. But I have gotten alot taller, and lost alot of weight. This growth spurt happened when I was around your age. You could be unknowingly loosing weight.
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stretch marks and body building
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You need to stop worrying about your weight and diet, it will make you un healthy. No stretch marks arent that common with people your age but because you are still young they will disappear. Like I answered your other question eat healthy, and exercise
You're growing, so yes it is common to get stretch marks.
You can use cocoa butter to make your stretch marks reduce.
To get rid of your stretch marks I recomend liquid vitamin E. You can buy it at you local pharmacy.