Treating stretch marks as soon as they appear is your best bet at beating them. So slather on some cocoa butter twice a day, and maybe even pick up some Bio Oil if it's not too much.
Unfortunately, as skinny as I am, I have them going from the back of my hips and stretching across my butt (I have basically no hips and a fairly small butt, so obviously this is frustrating). Because of my insane dysphoria, I NEVER looked at my body, in a mirror or otherwise, so I never noticed them when they must have developed in puberty, and now it's too late in the game to treat them. :/
If you're really willing to be proactive about this, try going on a diet. Start out by cutting out sugars. You'll lose a lot of weight just from that (my skinny self lost 10lbs in high school just from cutting sodas out of my diet, and I didn't even drink too much soda). Green tea that you make yourself is a GREAT way to help lose weight as well and also helps your skin.
But the bottled stuff has a LOT of sugar in it. You need to use the actual tea bags. You can even add honey to it to make it sweeter. Use that to get over any sugar cravings you may have (also, once you start cutting out sugar, you'll crave it less and less, and will be more drawn to "real" foods).
Also, cocoa butter helps make the skin tighter while you're losing weight, so loose skin doesn't become too much of a problem, even with faster weight loss.
Win/win.
Nipping this in the butt now is your best option, so hopefully you'll be able to do something about it! And even if they don't completely go away, they will fade over time, and become less noticeable over the years (although obviously addressing your eating habits and dropping the weight is necessary to keep from developing more). I don't know when mine showed up, but they're almost the same colour as my pale skin now, so they're not so in your face, and they're easier to overcome when looking in the mirror.
