I'm 14, I am not overweight 115 pounds and 5'7 I grow a lot but I don't know if that's the reason I have fairly good posture and I dont know how to get rid of them for good.
By red lines, do you mean stretch marks? If they are stretch marks- they will fade, but it's not likely they will disappear forever. You can help treat them by making sure to moisturize and use vitamin E oil.