Bob is eleven years old. One day, his friend Jenny said to him, "I'm going to have a birthday party on Saturday. Bob, can you come?" Bob asked his mother, and his mother said, "Yes, you can go. " On Saturday afternoon, his mother said to him again, "Now, Bob, when you go to the party, you must be polite , and don't ask for food until someone gives it to you. " "All right, Mom. " Bob said, and he went to Jenny's house by bike. There were many children at the party. They played games for about an hour, and then Jenny's mother gave them some food, but she forgot Bob and didn't give him any. After a long time, he took up his plate and said loudly, "Does anyone want a nice clean plate?" How old is Bob?
Nine. Ten. Eleven. We don't know.