I don't always have the same age in my head as the one I fill in on forms. When I turned sixteen, I still thought of myself as being fifteen. In fact, when I turned seventeen, I was only just beginning to think of myself as sixteen. On the other hand, I saw myself as fourteen for most of the time I was thirteen. I presently think of myself as almost nineteen.
I do know exactly how old I am, but when I fit myself into the age scale, I put myself at eighteen-and-three-quarters. If somebody is seventeen I think they're younger than me - which admittedly, they probably are. If somebody's nineteen, though, I think they're only a touch older than me, when they must be older by more than a year.
I think this is linked to the real insignificance of birthdays: that being seven years, three hundred and sixty four days old is not that different from being eight years old exactly. Still, seven-year-old and eight-year-old are useful descriptions, so we use them anyway. Sometimes we fit into different categories mentally, emotionally and physically.
Considering myself to be an age I'm not is a sort of automatic coping mechanism. All seventeen-year-olds do that? Well, that's because they're younger than me. Eighteen-year-olds act like that? Well, I'm only seventeen. As long as I keep on growing, I don't think it matters what age I'm at, but it's interesting to look at the numbers. I just have to remember to get the right age on the forms, or thing could get very complicated!