The answer is simple: as soon as you can. Ideally, you'd start saving in your 20s, when you first leave school and begin earning paychecks. That's because the sooner you begin saving, the more time your money has to grow. Each year's gains can generate their own gains the next year - a powerful wealth-building phenomenon known as compounding interest.
Here's an example of what a big difference starting young can make. Say you start at age 25, and put aside $3,000/year or $250/month in a tax-deferred retirement account for 10 years - and then you stop saving - completely. By the time you reach 65, your $30,000 investment will have grown to more than $472,000, (assuming an 8% annual return), even though you didn't contribute a dime beyond age 35.
Now let's say you put off saving until you turn 35, and then save $3,000/year or $250/month for 30 years. By the time you reach 65, you will have set aside $90,000 of your own money, but it will grow to only about $367,000, assuming the same 8% annual return. That's a huge difference.