According to the
law, at
eighteen you're an
adult.
But I personally feel there is no outside authority that can tell you when you are an adult. It's when you decide you are one. That doesn't mean at some time you decide you're an adult, and then for the rest of your life you are. You can always stop being one, whenever the mood hits you to be a kid again, or maybe just a teenager.
When sitting down and taking care of the bills or your taxes, you tend to be an adult. Often at work, depending on your job and who you're around, you're an adult.
I try and spend plenty of time not being one. I feel I've gained the maturity to know when I have to be one, and to do so, but I still have that spark which lets me play tug-of-war with the blankets on a silly Sunday morning. Or to totally blow off doing some cleaning in the apartment because I'd rather sit and play pinball or go in-like skating.
It's your mind. You currently have to grow older. It's a good idea to mature. But growing up is entirely optional.