It's when you stop caring about whether other people think you're mature or not. This isn't a snotty answer, I mean this seriously. When you're confidant enough in yourself that you go forward without worrying if some person you hardly know thinks you're mature then you're mature.
Not that it really matters, though.
Actually I would say that is the opposite of maturity.
So, you think maturity is worrying all the time whether people you hardly know think you are mature or not?
I'm not sure I agree with Sidious, but his reply makes a lot more sense than yours
And anyway, you are both wrong. Maturity is when you have all your tattoos painfully lasered off to prove how mature you are, apparently.