I would probably limit the description and show the character's environment, how he adapts to it, and how is treated. Age can be revealed by simply having him go to school. Some taunts by other school kids could show if he is overweight, very skinny, wears glasses, has an unusual hair style or color. Just keep it to one or two attributes. Athleticism or lack of it could be revealed by having the character go to gym class and be either the first one or the last one chosen for a team. Simply put yourself inside the character and imagine what a day in the life might be like. This will highlight the key physical characteristics that the reader needs to know. For me personally, I tend to favor very minimal character descriptions and I find that when reading, I forget most of the descriptions given at the start.