Is there any reliable way of using hormones to decrease breast density
in human females under the age of 30?  And what kind of hormones would
those be?  I am interested in mainly non-experimental kinds of therapies

breasts are dense at that age: it's normal. If by dense you are
referring to the difficulties associated with interpreting mammograms
or ultrasound in dense breasts, there's really no approved treatment.
It possibly could be done by doing any of several things to lower,
block, or counteract estrogen. Such as taking testosterone, danazol,
or tamoxifen, etc. However, the side effects of any, at the level that
likely would be required, would be significant, including stopping
periods, developing acne, affecting libido, etc. It's not, as far as I
know, an approved or studied use of such drugs.