This depends on how much you think your children's education will cost. The best way is to start saving before they are born. The sooner you begin the less money you will have to put away each year.
Example: Suppose you have one child, age six months, and you estimate that you'll need $120,000 to finance his college education 18 years from now. If you start putting away money immediately, you'll need to save $3,500 per year for 18 years (assuming an after-tax return of 7 percent). On the other hand, if you put off saving until the child is six years old, you'll have to save almost double that amount every year for twelve years.
Another advantage of starting early is that you'll have more flexibility when it comes to the type of investment you'll use. You'll be able to put at least part of your money in equities, which, although riskier in the short-run, are better able to outpace inflation than other investments in the long-run.