"Do stretch marks go away with time? Is there anything you can do to get them to lighten?"
Firstly, a disclaimer: I don't actually have much personally informed knowledge/experience regarding stretch marks—that's not necessarily to say that I haven't had any, but they've simply never bothered me much, so I haven't really paid much attention xD).
That said, it's a good question, and it deserves an answer, so after some research, here's what I've gathered.
Stretch marks (as you likely already know, but for the sake of any other readers, and just to clarify that we’re on the same page) are thin streaks or marks in the skin, often reddish in color, which are caused by rapid growing (or stretching of the skin, hence the name)—they occur frequently in kids during or after growth spurts, and in adults as well; stretch marks are very common for pregnant women. Common parts of the body for stretch marks to develop include the breasts, thighs, hips, and backside, as these are areas that are especially likely to experience rapid growth/changes/stretching, although they can occur in other parts of the body as well.
While they typically lighten with time, it does seem that, once formed, most stretch marks don’t entirely disappear. However, they do fade and become less noticeable, until they may well end up all-but invisible except for as pale, barely detectable lines.
If you search for ways to “get rid of” stretch marks, you’ll find that many companies do market various products—creams, ointments, etc.—that, supposedly, eliminate stretch marks… but I’d advise being very wary of these products. On top of often being very expensive, the unfortunate fact is that most of these so-called “cures” are really quite ineffective… and that’s not even taking into account the risk of other side effects that such creams might have on your skin or body in the short or long run. Stretch marks themselves are benign, meaning that, while they may not be something you like seeing in your skin, they won’t do you or your health any harm either.
Short of going to see a dermatologist (skin doctor) or get some sort of cosmetic surgery (not recommended, given that, as I see it, the costs and risks of complications of such “treatments” far outweigh the benefit of getting rid of something which doesn’t cause any problems in the first place except, possibly, aesthetic ones; and either way, I believe most doctors wouldn’t recommend such treatments for kids or teens regardless), your best option is probably just to hang in there and try not to let it get to you while you wait for the marks to diminish, I’m afraid. If they really bother you, some people choose to cover up their stretch marks with things like body makeup—though again, to me, this doesn’t seem worth it. (Some kinds of makeup are extremely unnatural, and can actually end up harming your body in the long run.)
As you go through puberty and adulthood, you’re likely to develop new stretch marks throughout your life, and your old ones are likely to fade and lessen. In the meantime, remember that, no matter how much you might wish your stretch marks away, you’re not alone; at least a third of kids (quite likely, more—I’ve found different estimates/statistics from different sources) will develop stretch marks in some part of their body at one point or another, whether during puberty, adulthood, or both. Almost always, stretch marks aren’t an indication of any sort of underlying problem with your body and don’t hurt, so they’re perfectly natural and harmless, if sometimes annoying.
My best advice regarding stretch marks? (Note that this last bit is based both on research and on personal experience.) It can be hard, but do your best not to worry about them.
Hope this helps!