I don’t know where I heard or read it, so take my explanation with a grain of salt. Most poisons taste bitter, so avoiding bitter tasting things means avoiding poisonous things. Since children have a smaller body weight, the lethal dose is smaller for them. As go grow older, your body mass increases and so does your tolerance to poisons. But I am not sure why we would not want to avoid poisons in general even if they were not lethal any more.