In "Master Harold". . .and the boys, Hally must have been around ten years old when Sam made the kite for him.  During the conversation at the tea shop, Hally recalls his memories from the "old Jubilee Boarding House."  He often hid in Sam and Willie's room when his mother was looking for him so that he could talk to Sam and Willie and play games with them.  At one point, Hally says, "No joking, if it wasn't for your room, I would have been the first certified ten-year-old in medical history."  Shortly after that, Hally recounts his best memory from that time, and the memory includes Sam making the kite for him and them flying it together in the park.  So Sam and Hally already had a bit of a relationship developed by the time the kite was made, so Hally likely was still ten-years-old at the time.