There has been some interesting research done that says that babies as young as 5 months could tell the difference between groups of 1, 2 and 3 objects* and that adults can determine numbers up to 5 without counting. Adults can even determine amounts of 10 and under by seeing them as amounts of 5 plus 1-5, and rapidly doing the calculations.
By using a specially made abacus, you can teach children math skills, cashing in on this natural ability, and they can build on this and later can picture the abacus in their heads while doing mental calculations. This abacus is set up in rows of ten beads, five of each of two colors. Quentin saw how it is easy to tell 5 from 4 because there is one in the middle with even amount on either side. We also looked at tally marks and how the fifth mark is like drawing the thumb across your hand.
* "Addition and Subtraction by Human Infants," K. Wynn, Nature, August 27, 1992, Vol. 358, pp. 749-750.