What's an Adult?

I found this on facebook and it got me thinking. First about the meaning of the word man. Easy enough. Male adult. And woman? That's a female adult. Easy enough if you understand gender. But what's an adult?
For starters, it's one of my least favorite words in the English language. Maybe someday I'll come up with a stupid euphemism like "differently aged". But until then I have to use it because the only synonym is grown-up, which makes you sound like a toddler.
Adult is the root of words like adultery and adulteress. Adult video. Adult content. Adult audiences only.
So adult means sex.
But then I hear it used to mean mature or competent.
So adult means capable.
I've consulted several dictionaries, and if you care enough you can consult them too. As far as I can tell, adult is a noun meaning "one who can have sex without looking like an idiot in front of society."
Sound off in the comments if you disagree.
I'm sixteen. A woman in the technical, biological sense. I could go make babies right now if I felt like it, but I won't. I don't feel like it. And I don't think it's right. Not now and not when I'm twenty six.
I'm old fashioned. I believe that sex outside of marriage is wrong, partly because of my religious background and partly because I've seen it screw up so many lives.
But most people don't care. Go ahead, you can watch adult videos with adult content for adult audiences only. So long as you're an adult. 
I don't like porn. I don't believe it's appropriate for anyone. You can't use the maturity argument here. You can't say hormones affect a teenager's perception of porn the way you say cognitive abilities affect our perception of driving. And please, don't try to use the "We're more experienced" argument.
Maybe the word adult doesn't mean anything. Just because you're capable of creating a child doesn't mean you're capable of raising one. It doesn't mean you're capable of driving a car. It doesn't mean that you're smarter than this woman.
It doesn't mean you can do an honest day's work like the guy on the left.
So you decide. Who is the real man?