Short answer: The earlier, the better ...
The earlier you save, the more time your money has to grow. This is the magic of compounding—when your returns earn more returns and so on. You can open a 529 and make the most of the time you have as soon as the beneficiary has a Social Security number!
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Here's an example
Let's say you start saving for college when your child is born. You invest in an account and save $25 a week for the first 9 years of their life but then stop—for a total investment of $11,700. If your account earns 6% a year, you'll have about $26,750 at the end of 18 years.
Now let's say you wait 9 years before you start to save, and then save the same $25 per week until your child is 18. Factoring in the $11,700 investment and 6% return, you'll have accumulated about $15,800 by the time they go off to college.
As you can see, you'll earn almost $11,000 more for college in the first scenario, thanks to the power of compounding!
Saving earlier means you'll have more for college