Mentality

It's something we all do, whether or not we want to, it's growing up.
We start out as young babies, knowing nothing, and we work our way through out the years to become wise old people. How is age based? Sure, you could go with how many years people have been alive. But I'm sure some day in the future this won't get you very far. Is it looks or actions or even how much knowledge we know?
If it's based on knowledge then there are alot of people out there who have some growing up to do. I believe that a persons first hint at age is how a person looks, if they have white hair and wrinkles then we believe them to be old. So you could go with looks.
What really matters I believe is how a person handles situations in life. This is what I think makes us grown up or not. There are instinces in life where people are forced to grow up sooner than other people, but hey that's life for you.
If someone where to judge a person on how they handle the situations they find themselves in then I think you would have a much better guess of their age, and a better understanding of that person.
True, this may not always be the case, but there will always be exceptions.