The “0th” birthday sign was a literally correct age for the human who had arrived a few days before.
Technically, in most cultures, a person is considered by 0 years old at the moment of their birth. It’s also reflective of the age many records use to list individuals within the first 12 months of their birth.
And it’s difficult to get mixed up on whether someone is 0 years old or 1 year old–even if there is no record of their birth. But as that person ages, it’s easier for a record to be a off or inconsistent with other records. It’s more difficult to be off by much when someone is under the age of 3. When that person with no birth record has aged to their seventies, it’s much easier to be off a year or two on their age.
The older a person gets, the more their age is to vary especially if they do not have a record of their birth and someone else is determining their age for them.