I'm fifteen and I have stretch marks on my butt, thighs and breast. usually you get stretch marks because you loose and gain weight frequently whether you stay the similar weight or not. your skin stretches. you can be skinny to have these or you can be any other size, it doesn't necessarily mean you're and I quote "thick" which is what people my age associate it only with. you can try applying cocoa butter onto your stretch marks it'll help them fade out a bit is what I've heard but quite frankly I haven't tried it because I like my stretch Marks. don't be shy of them!
Stretch marks never fully go away. Over time they will fade. You will look great in a bikini. Stretch marks are a sign of growth. Growth that the skin isn't used to therefore it's stretched beyond its means in a short period of time and the result is stretch marks. Stretch marks are more common than you think! Happy growing pains.