I will be 66 (my full retirement age) in September, 2018. Upon reaching age 66 I will have only 23 years of employment that will get used to determine my social security benefit and the rest of the years are zeros. If I continue to work until age 70, will those last 4 years replace 4 zeros? Will I then have 27 years of employment that will get used to determine my social security benefit? Or do only the years of employment before full retirement age replace zeros? Thank you very much.
Yes. Your yearly earnings can be used to calculate your Social Security retirement benefit rate regardless of your age at the time you had the earnings. And, your retirement benefit rate is based on an average of your highest 35 years of wage-indexed earnings (https://www.ssa.gov/pubs/EN-05-10070.pdf). So, it sounds like you could work for another 12 years and still be replacing zero earnings years that would otherwise be used in the calculation of your benefit rate.
You would not want to wait past age 70 to claim your retirement benefits, though, because you wouldn't earn delayed retirement credits after reaching age 70. Your benefit rate could, however, be recalculated and increased after any future year in which you have Social Security covered earnings no matter how long you continue to work.
You may want to strongly consider using our maximization software to do your Social Security planning. Our software would not only permit you to explore and compare your various filing options, it also allows you to input your anticipated future earnings so that you can gauge their effect on your benefit rate.