This might sound like a dumb question but just looking for some validation… Is the age twenty one not the age of a legal ADULT? As in if someone tells you todo something and you don’t because you’re an ADULT capable of making your own choices/deductions absurd?
Depends. Who is asking you to do what? The right to decline does not guarantee a lack of consequences.
Are you still living in your parent’s house?
depends on the reason you don’t want to do it. if it’s truly an unreasonable demand, then that’s one thing. If you’re resisting just because fuck anyone who is trying to tell you what to do, then grow up. I mean, I’m 36 but that doesn’t mean I can tell my manager to fuck off just because goddamnit I’m an adult. well, so is he, and he can fire my ass.
Nothing in particular…only wanted other people’s take on it…
I think of 18 as the age of adulthood.
You still have to listen to your mom until you move out, if that’s what you’re asking.
Adults have a magical ability to not have to do what people tell them to do, without any repercussions, because they are ADULTS?
News to me.
My advice to the OP: You need to grow up, whatever age you may be.
The age of majority in the US is 18 though some states make it 19. The right to vote however is established nationwide as 18.
As others have said, you can always refuse to do something but part of being an adult is recognizing and accepting the consequences of your act. There isn’t a hard and fast age for that.
Agreed. I always find it a bit absurd when I hear about the drinking age being 21 in the US - you’re an adult for three years before you can take a legal drink?!? Weird.
Yeah, this too.
Since you won’t specify, then I’ll just say it depends.
I dunno. I think “They let you vote and enter contracts three years before you are a legal adult?” better matches the actual maturity level.
As I understand it, the idea was that they shouldn’t draft people that couldn’t vote, so the age was moved back. But the reason the military likes to draft 18 year olds more than 21 year olds is not “They are plenty mature at 18: take them then”. It’s because 18 year olds are a lot more malleable, on average.
You are an adult when people trying to tell you what to do doesn’t bother you anymore.
My house, my rules applies at any age.
I call bullshit. 
There’s no age of legal “adulthood”. There’s various ages when various legal rights and responsibilities kick in.
There’s no age when you can do whatever you want. If your spouse or your boss or a police officer tells you to do something, you’re probably better off if you do it.
Oh come on, people. Even if the question was phrased a bit naively, it’s a valid topic with a factual answer:
So good it bears repeating. 
Well in my experience adulthood doesn’t really begin for most people until their late 20s; you may be legally able to do stuff but you don’t really grasp the consequences of what you do until that age.
Don’t get me started. Adults, at 18, should have full legal rights.
I’m assuming there’s a triggering event with an exception. Like, the 21-year-old is living in his/her parents’ house/using their property in such a fashion as to cause them legitimate concern over their adult child’s conduct.
If not, the answer would be, “well, no shit.”