On a literal level, the Iron Age was a prehistoric time during which tools and other implements were made out of iron – but that doesn't really seem to be what we're dealing with here. Rather, the title Age of Iron comes out of Mrs. Curren's observations of how the time she's living through is marked by hatred and hard-heartedness.
Let's take a look at the first instance in which the phrase "age of iron" pops up, shall we? Mrs. Curren has just watched Bheki beat the smack out of Vercueil with his belt, and she admonishes Florence for allowing it to happen:
Children of iron, I thought. Florence herself, too, not unlike iron. The age of iron. After which comes the age of bronze. How long, how long before the softer ages return in their cycle, the age of clay, the age of earth? A Spartan matron iron-hearted, bearing warrior sons for the nation. (2.100)
Mrs. Curren thinks of iron as a symbol of the hard-heartedness and hatred she's seeing all around her. By characterizing the time that she lives in as an "age of iron," she seems to be saying that this kind of hatred doesn't belong to just a few individuals; rather, it characterizes an entire generation. She witnesses adults and kids alike engaging in acts of hatred of violence, and it doesn't seem like it will stop any time soon. Still, different ages in history have come and gone, so she does seem to suggest that it is possible for the age of iron to end and for a new age of love and peace to begin (but maybe that's just wishful thinking).