What was the significance of it? Did they celebrate their birthdays somehow? How did they do so before the Gregorian calendar became standard?
Edit: What are the theories of how Biblical ages were determined?
This is more of a question for r/AskAnthropology. People have known how to count for far longer than we have a historical record for, so other means are needed to find the answer to this question. I have some thoughts on how to get an approximate answer, but they're not appropriate for this sub.