I still have 9 months until I turn 21 but i've found a killer deal on some NIW preban magazines for the CZ75 and i'd like to snap them up before the opportunity passes me by. Is it legal for me to buy pistol "paraphernalia" such as hi-cap mags before I am 21?

No, unless you live in an area with some kind of lacal restriction.  I am not a lawer but I would say you are safe on this one.]

Talon_G32

Yes, it is completely legal for you to buy handgun accessories, just not the actual handgun itself.

The handgun purchasing prohibition only extends to buying a handgun from a dealer. In most states that I am aware of, it is perfectly legal for you to purchase a handgun from a private individual or for someone to give you a handgun as a gift as long as you are not otherwise prohibited from posessing a handgun.

I got my first handgun at 18. Ruger P94DC in .40 S&W.