But nobody is doubting that these people exist, or saying that their documents were faked maliciously. Back in my grandparents' time, they had a child, then a few years later they went to register the child, the clerk asked "how old is she?" and they said "7 or 8" when she was 11, for example.
My grandmother never knew her birthday, or her birth year. We just estimated, even though she was very much real.
Depends on where you live, I suppose. I know a lot about how it was done back as far as my great-greats and sometimes older, because the church records are meticulous and written with a lot of comments, and notes if there's been any kind of delays. And never would a child be registered years after birth.
It definitely depends where you live. My great grandparents wouldn't make the day's trek down the mountain to the city and sleep in the open just to register a child.