For what reason would a young girl (13 - 20 years old)(not having children) have drooping breast?
do you mean as in "sagging"? if that's what you mean than it may be because of the size. If you have larger breasts It happens sometimes and it's natural, ever heard of gravity? the best way to prevent it is the regular use of a good bra.
what class is this?
Weight loss can do it. Also some girls are just built that way. It might be hereditary.