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!
As was mentioned in the previous post, Scout reveals her father's age in Chapter 10. In Chapter 10, Scout criticizes Atticus for being older than her friends' parents, and she mentions that he is nearly fifty years old. In Chapter 12, Scout learns several facts about Calpurnia. When Jem asks Cal where she went to school, Cal tells him that she never attended school. Cal says that Miss Maudie's aunt, Miss Buford, taught her how to read. Jem then asks Cal, "Are you that old?" (Lee 76). Calpurnia responds by saying that she is a few years older than Atticus but is not exactly sure how old she is. Calpurnia explains to the children that she was never told the exact day she was born and simply celebrates her birthday on Christmas. Assuming that Atticus is "nearly fifty," one could surmise that Calpurnia is in her early fifties, approximately 50 to 53 years old.