This is usually done when they are a few days old. But there is actually no way to answer you with certainty. I once had a broody suddenly decide that the 3 week old chicks who had been growing up next to her (separated by chicken wire) decide the chicks were hers. She started squawking and pacing along this fence, as if she were a mama who had been separated from her "babies." So I tried them together, and she acted like a new mama for a month, corraling them, offering feed, protecting them, etc. It was a bit comical since they wanted their independence and weren't very well behaved "children," lol.
So, you just never know.