I had been reading that adult is all about porn. So, when your genitals grow big and strong, you mature into an adult ?

been reading that adult is all about porn

Don't know what this means, but literally yes, you're an adult when you physically finish puberty and brain development more or less. I know for biological men we would usually anchor this to "willy is as big and hairy as it'll ever get. Sigh. Guess I have to pay taxes, now!"