Like so many other things, I don't really see the point of waiting to teach certain concepts until a particular age or grade. When kids are ready to get it, they're ready.
And they're usually ready before most people think they are!
Right now my kindergartner and 3-year old are at about the same academic level, (Dash is way ahead with his gross motor skills, storytelling, humor, etc. but a bit slower in fine motor skills and attention span )
I simply start by telling the boys that the hungry alligator wants the side with more food.
He always eats the bigger number. Simple, right?
(below, my 3-year old LittleLad draws his "monster teeth," to eat the number 9)Within 30 minutes they progressed from manipulatives (poker chips and number rods, in this case) to number puzzles playing cards, dominoes, Skittles...