20 yrs old with six months experience and a bootcamp certificate sends a few signals. You don't have much experience, but you are eager and motivated. Your age also communicates that you are likely single without children, no mortgage or big external commitments, so likely to work 12+ hrs/day six days a week for free pizza and beer. You are probably going to do what you're told and not rock the boat (for a while anyway). If they have you on an H1-B you have a big incentive not to make a fuss.

Part of the reason companies prefer to hire younger people is their energy and enthusiasm, but another part (that they can't and won't spell out) is that older people have families and commitments and may want to actually have a work-life balance and not just hear about the company's "committment" in some HR feel-good speech.

Youth is definitely more a plus than a minus in SF/Silicon Valley but you need the experience, some solid accomplishments you can point to. Or you need good contacts that can tell you about the unadvertised jobs (which are most of the available jobs) and get you in the side door.