“Even an older child?” asked Aharon. “I’m already bar-mitzvah!”
“The Gemara [Eruvin 79b; B.M. 12a-b] differentiates between younger and older children,” replied Rabbi Dayan. “There is a question whether this means before and after bar-mitzvah, or financially dependent and financially independent. Therefore, since you are still financially dependent, it is preferable to have someone like Chaim, who is not a household member, acquire the food for the community. If we had married children here, they could also accept the eruv.” (C.M. 270:2; O.C. 366:10)
“Wow! We both learned something new about eruv tavshilin!” exclaimed Chaim. “And just think, it even ties into monetary halacha!”