Great question. I've dealt with this once before. This patient was 21 as well and came to visit me from out of town. Because she had almost no breast tissue at all, it was clearly the better implant to use (silicone) for many reasons. I sent her off to return to me in a year. Needless to say, she came back with really big saline implants that just didn't look right, but she made all of the choices and signed all of the paperwork. I may be wrong, but it seems that all that paperwork is not necessarily intended to protect the patient.
I think that some would do your surgery with silicone implants despite FDA recommendations because its not illegal. This is an elective cosmetic procedure and those are the product recommendations.
Although I don't think the FDA necessarily makes all logical choices, their intention was originally meant to protect patients. Making you sign a series of consents where you accept all blame doesn't seem right to me but you won't struggle to find someone that's perfectly fine with it.
I don't regret not having done this patient's surgery, in fact, she's already talking to me about switching them out for a different size. My suggestion is for you to talk to your plastic surgeon and see what he recommends. If he tells you to wait and you bonded with him and trust him.. you should probably wait. Keep us posted on what you ended up doing, I'd be very interested in knowing.