I don't think it's necessary to begin with. Whether you are seen as an adult by the government or not, that's just the 'officiality', so to speak. You're an adult by law now. If that makes sense. Your thoughts, opinions, body, etc. doesn't simply change because you happened to be classified as an adult.
Meaning to say: Nothing has changed

So if you want to tell your parents regardless, go ahead, but don't let the motivation be just because you are classed an adult x3
If you feel it's your own personal business, then keep it to yourself. If you feel you want to tell your parents, then by all means ^^
It's really up to you, adult or not.