True, if one is talking about retiring before 40. You just don't have enough years in to get all of the more easily grabbed portion of the SS benefit.
However, if one is talking about retiring late 40's or early 50's, which I definitely consider FIRE, one has likely put in most/all of the key years for Social Security. After 25 years of working a white collar job (I'm not talking big money like finance - just something solid like engineering), you are near/past the 2nd "bend point" and have reached the point of diminishing returns. More years don't add a lot to the SS benefit.
1. Assuming you worked less than 35 years... Calculate your monthly earnings. Your earnings are listed in your SS statement and are indexed for inflation.
2. Let's say your average (indexed) salary was $80k and you worked 25 years (age 48, say). That is a total of $2M over your career. Divide by 35 years. $57k/year. Divide by 12. $4,762/month. That's your AIME - average indexed monthly earnings.
3. Check where you are vs. the
Benefit curve. AIME on the horizontal axis, SS monthly benefit on the vertical. If you can, you want to get further to the right than "Joe".
4. Why? Your SS monthly benefit (age 67) is computed as: 90% x (your first $895) + 32% x ($895 to $5397) + 15% x (monthly earnings over $5397). You want to maximize the 32% chunk.
Once you are over $5397 AIME, bumping up your average monthly salary by working more years towards 35 years just doesn't impact benefit number that much. Right at the 2nd bend point, your monthly benefit would be $2250. After that, it really creeps upward.
For the worker in the case above, I'd definitely recommend working three more years to reach the 2nd bend point.