Aside from the obvious causes like pregnancy and rapid weight change, a few other reasons why stretch marks might appear include: Genetics or a hereditary condition. Having overly dry skin. Having previous damage or injury to the skin.
Fluctuating hormone levels seem to play a role. You may also have a higher risk if people in your family get stretch marks. If you develop stretch marks, you're most likely to do so during these times: Growth spurts that happen in puberty.
People get stretch marks when the middle layer of the skin, called the dermis, becomes stretched. They appear on the thighs either when a person gains weight or muscle around the thighs or when they lose weight in this area. Growth spurts and pregnancy can also cause stretch marks on the thighs.
Stretch Marks are a Normal Part of Puberty
 It is normal for teenagers to get some stretch marks. The marks occur when a person experiences a significant amount of growth or weight gain in a short period of time, such as during puberty. Getting stretch marks does not necessarily mean a person is overweight.
Stretch marks usually fade over time but may not disappear completely. Many creams and lotions claim to prevent, reduce or remove stretch marks. But there's very little evidence these work. There are some treatments that may help make stretch marks look better, but they will not get rid of them.
Development of a stretch mark
 The skin is stretched as a consequence of underlying tissue expansion. There is an inflammatory reaction in the skin that is responsible for the characteristic red or purple colour. The inflammation eventually fades and is replaced by scar tissue.
Thigh fat removal can happen naturally through proper diet and exercise. A diet rich in fruits, vegetables, lean proteins, and fat-free products can help. You should also consider switching from processed grains to whole grains. Do your best to avoid refined and processed foods and alcohol.
In general, stretch marks from weight loss fade or disappear completely on their own. Sometimes rapid weight loss can actually cause stretch marks because of excess skin weighing down on the healthy skin.
ANSWER: It is possible that your son developed stretch marks during his growth spurt. When rapid growth or weight gain occurs, especially during teenage years, it is common for stretch marks to appear in boys and girls alike. There are many changes during the teen years.
Stretch marks are fine lines in the skin that happen when quick growth or weight gain stretches the skin (like during puberty). Skin is usually fairly stretchy, but when it's overstretched, the normal production of collagen (the major protein that makes up the tissue in skin) is disrupted.
Radiofrequency Treatments
 Allure reports that RF treatments use energy to heat the skin's deeper layers, stimulating collagen and elastin production. This process tightens the skin and reduces the appearance of stretch marks by improving skin elasticity and texture.
As fat cells increase, they push up against the skin. Tough, long connective cords pull down. This creates an uneven surface or dimpling, often referred to as cellulite. Cellulite is a very common, harmless skin condition that causes lumpy, dimpled flesh on the thighs, hips, buttocks and abdomen.
What does “skinny fat” look like? Phenotypically, they look lean and healthy, but when we check them out they have high levels of body fat and inflammation. This ectomorph body build has hidden high levels of body fat. They appear thin and flabby.
Cause: Weight Gain
 You can get stretch marks when you're young and your body is changing quickly. The more weight you gain and the faster you gain it, the more likely you are to have stretch marks. Putting on lots of muscle mass quickly, as bodybuilders sometimes do, can also cause them.
Aside from the obvious causes like pregnancy and rapid weight change, a few other reasons why stretch marks might appear include: Genetics or a hereditary condition. Having overly dry skin. Having previous damage or injury to the skin.
This discoloration is often a sign that something is affecting the circulation in your legs or the health of your blood vessels. While it might initially seem harmless, purple discoloration can sometimes point to underlying issues like poor circulation, vein damage, or even more serious conditions like blood clots.
Stretch marks are often caused by gaining weight rapidly over a short period of time. Avoiding rapid weight gain and weight loss can help prevent stretch marks. If you need to lose weight, you should do it slowly by: eating a healthy, balanced diet.
New stretch marks will look slightly raised with a red or glossy colour. Over time, they eventually fade to white and start to look slightly depressed. Older stretch marks may look like scars.
Losing more than 1 or 2 lb (0.5 to 1 kg) a week is not safe for most people. It can cause you to lose muscle, water, and bone density. Rapid weight loss can also cause some side effects including: Gallstones.
Genetics and a lack of exercise are standard reasons, but for women, thigh fat can also commonly develop for estrogenic reasons. The amount of estrogen receptors in the thighs and the effect the hormone has on fat pads in the thigh area can also be primary causes.
Try simple exercises like lunges (both forward and side lunges), squats, and leg lifts. Gyms and sports clubs have resistance machines that can help you tone up specific areas, like your upper and inner thighs.