The passage that you are looking for can be found towards the end of chapter 12, and the answer you have above is correct. And, at the beginning of chapter ten, Scout says that Atticus was "nearly fifty", so we have to figure that Cal is a bit older than that. Specifically, in chapter 12, Scout asks exactly how old Cal is. Cal responds:
"I'm older than Mr. Finch, even...not sure how much though. We started remberin' one time, trying to figure out how old I was-I can remember back just a few more years more'n he can, so I'm not much older."
This might seem strange to us, who celebrate the exact day and year of our birthdays with exuberance and excitement, keeping fastidious track of exactly how old we are. However, in Cal's culture, many of the people there couldn't read. So, when babies were born, it wasn't like they had a calendar that they could refer to to figure out the exact date. Plus, in poorer communities, when you are born is less important; people didn't have money to celebrate birthdays anyway. People might remember the season or time of year (Cal just says that she celebrates on Christmas, because it's easier to remember that way), but the daily grind of life kind-of took over such things, and money was short, so birthdays got lost in it all. I hope that helps!