Thirty. Something just "clicks" when you hit thirty, and you tend to see everything through an entirely different lense.
Back when I was twenty-five I was told this by a guy who had just turned thirty-one. I laughed at him, but lo and behold, when I turned thirty something happened upstairs.
I'm sure it's different for everybody, but for me it went in stages, and I would not say that you really have a proper grasp on life until you have been a legal adult for ten or twelve years and had time to explore yourself and life in general.
Either way you look at it, I am positive that every ten years or so you will look back and wonder how you could have possibly been so naive, and even wonder how it is that you are still alive.
Age makes for experience, and experience is the bringer of wisdom. Ten years is a long time...