Some time before five, maybe four, maybe three, you could start "skip counting." By twos, fives, and tens. Doing this and filling the gaps, by kindergarten, one

Some time before five, maybe four, maybe three, you could start "skip counting." By twos, fives, and tens. Doing this and filling the gaps, by kindergarten, one of my kids could start multiplication. By first grade, she had a working understanding of fractions and the arithmetic around them.