【小学·阅读理解·A Note on the Desk】
Yesterday morning Sam found a small note on his desk. It said: "Meet me under the big tree after school. — A friend."
Sam could not think all day. Who wrote it? He looked at his classmates, but everyone just smiled and said nothing.
After the last lesson he walked to the big tree. Under it stood all his classmates and his teacher. On a table there was a big cake with eleven candles. They all shouted, "Happy birthday, Sam!"
Sam had forgotten his own birthday. That afternoon he ate cake with twenty friends and said, "This is the best note I have ever got."

第 2 题：How old was Sam?（　　）