From IRS Notice 87-13:
A distribution to an employee from a qualified plan will be treated as within section 72(t)(2)(A)(v) if (i) it is made after the employee has separated from service for the employer maintaining the plan and (ii) such separation from service occurred during or after the calendar year in which the employee attained age 55.
Section 72(t)(2)(A)(v) is the section that establishes the age-55 exception to the early-distribution penalty. It's clear from this that it does not matter when during the separation-from-service year the employee reaches age 55. The employee can have their 55th birthday that year before or after separation from service.
