With anyone 65 years or older eligible to start receiving checks.
The 1875 date almost certainly comes from that. I wouldn't be surprised if someone set it as a default for anyone they didn't have a birth date for because you could safely assume someone was older than that if they were receiving social security when it first started _or_ because there are some of those initial payments that truly were never discontinued.
With anyone 65 years or older eligible to start receiving checks.
The 1875 date almost certainly comes from that. I wouldn't be surprised if someone set it as a default for anyone they didn't have a birth date for because you could safely assume someone was older than that if they were receiving social security when it first started _or_ because there are some of those initial payments that truly were never discontinued.