The Gemara quotes a Mishna that one who is not obligated to do a mitzvah cannot do so on someone else’s behalf, for instance a katan cannot recite birchas hamazon for a gadol. According to the view mentioned later that hirhur kedibur (contemplation is equated with actually reciting) why can’t the gadol fulfill his obligation by merely listening to a katan and contemplating his words?
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The difference would be if the listener does not understand Loshon Hakodesh, where hirhur kedibbur would not be effective, but the listener can be yotze from a godol but not a koton.