From this language we learn that even if he cannot eat, for example he has a weak constitution, still since others his age can eat, we need to distance ourselves (from the extriment to read shema). This requires analysis as we have not found this in any posek, and in addition the gemara indicates that if he was older, even though he cannot eat a kazayit in the alloted time of a Peras, we still need to distance ourselves from his excrement. This implies that for a child if he cannot eat it then it would be permissible (to say shema). Therefore it appears to me that the law is that if someone his age can eat, the assumption is he can also. And if we know that he cannot because he is weak etc, even if he is 3 or 4 years old we don't need to distance ourselves. And if he was an adult, even if he cannot eat, we still need to distance ourselves. The Bach adds that we also don't distance ourselves from (extrement) of a minor if we know that that specific extrement did not come from grain, for example he hasn't eaten grain for days like Rashi writes. The poskim write like this. And if we know he has never eaten grain, even if he is a few years old, you don't need to distance. Rashi writes that all 5 grains are called dagan and the feces smells. In yoreh death 265:8 it seems that it is better to distance oneself from extrement, even of a minor, even if he is 8 days. The Maharshal says it is not proper to read Shema next to a baby as it is probably dirty from garbage. End of his words. It seems to me for a different reason, that they don't clean themselves properly, and feces around the anus, even though it is covered is forbidden (to read by it). Look at siman 76:6