From MoneyWatch.com:
 
As April 15th approaches, CBS MoneyWatch  is publishing daily tax tips. See the full list here, and be sure to  check back frequently for the latest advice from our experts.
 
When it’s time to take money out of  your 401(k) or IRA, the magic number is 59 ½. That’s the age at which  you can withdraw money from a retirement plan without handing the IRS  a 10% bonus on top of the regular taxes you will owe.  Everyone  knows that, right?
Judging from the mail I get, everyone  does indeed.  But what not everyone knows is that the age 59 ½  rule has more loopholes than Tiger Woods’ marriage contract. For most  practical purposes, the penalty-free retirement age in a 401(k) is 55,  and it can be lower still for an IRA. Early retirement, medical emergencies,  job loss, early retirement, college education, a home purchase-all qualify  as exceptions that can make your retirement money more available than  you thought.  
Here’s how it works:
 
Separation from service after age 55  (401(k) only) your 401(k) money becomes yours without a penalty if you  leave your job after age 55. It doesn’t matter whether the departure  was your idea or your employers’, or whether you permanently go fishing  at that point or find another job the next day. You just need to “separate”  from your employer.