What exactly do you have to do in order to qualify as a teenager? In English speaking countries, most people would say, "Turn thirteen," because it's the first number with the "-teen" suffix. At my friends' thirteenth birthday parties, I've heard their parents say, "Great. Now she's going to be all sassy and rebellious with me." As if it's an overnight transformation. As if we're funny little caterpillars who go to sleep one night and wake up ugly moths the next.
From experience, I'd say it's much more a mental transformation than physical or numerical. I'd did a science fair project on the power of suggestion a few weeks I go. I showed kids, teenagers, and adults pictures of various faces and then asked them to write down all the ones they remembered. While they did this, I'd mention two faces-a clown and an asian guy with a scar-that weren't actually on the paper. Most of the kids believed me and wrote down the descriptions. The teenagers argued, but wrote them down so I'd shut up. The adults didn't fall for it.
When we're little, we innoccently believe anything we're told. Then we grow up. We teach ourselves to think. We consider the idea that we're not just misinformed and disobedient, we're right. We try to argue, try to win, and when we can't, we submit and settle for thinking rebelious thoughts.
Then there's a day when we don't give in, because we're sick of it and we know how to win now.
That's called maturity.
Shakespeare once said, "I would there were no age between ten and three-and-twenty, or that youth would sleep out the rest, for there is nothing in the between but getting wenches with child, wronging the anciently, stealing, fighting. I will now go forth to write something poetic and complicated youth four hundred years from now will be forced to read in school."
At least, he said the first part.
If everybody did sleep away the "in the between" years, we'd have a world run by ten year olds. Tall ten year olds with facial hair, but still ten year olds.
A world of caterpillars, soaring above the milkweed plants, but never appreciating them.
Most people may see the teenage years as the Dark Ages. I'd say they're the Renaissance.