The age that one seems to be is subjective. In some contexts, one can appear to be very juvenile, while, in other contexts, they appear to be very mature.
Because of my short stature and round features, I am often presumed to be much younger than I actually am...until I begin to speak. My extensive vocabulary leads others to believe that I am a bit older than I am. I am currently seventeen years old; within this year, people have guessed that I am between the ages of twelve to twenty-two.
Age is irrelevant to me, so I tend to ignore the age under a person's avatar.
It does not surprise me that you are fifteen. Then again, teenagers are extremely varied creatures.