While written determinations are not to be used a precedent, they do reflect how the IRS interprets the tax code. In several written determinations (Private Letter Ruling Number: 9219042, for example) which make reference to Revenue Ruling 83-57, the IRS indicated that the "balance to the credit" is determined at the time of the first distribution after reaching age 59½.
The "balance to the credit" of an employee for purposes of section 402(e)(4)(A) of the Code generally includes all amounts credited to his account and payable because of his death, disability, or separation from service, or all amounts credited to his account as of the date payments from the plan commence after the employee attains age 59 1/2.
It's this method of determining the "balance to the credit" that causes reaching age 59½ to be a singular triggering event.
