We know from the text that Ona is only fifteen as the novel opens on her wedding day. Jurgis is a little harder to piece together. He has only come recently to America, and spent 25 years in the Lithuania with his father. While we don't get his exact age, we learn that he is young and strong, in the prime of life, so probably not much more than twenty-five. He is delighted to get a job in the meat packing plants of Chicago. We learn that:

Jurgis talked lightly about work, because he was young. They told him stories about the breaking down of men, there in the stockyards of Chicago, and of what had happened to them afterward—stories to make your flesh creep, but Jurgis would only laugh. He had only been there four months, and he was young, and a giant besides. There was too much health in him.

The characters' ages are significant. Ona is a fragile, innocent young woman who has led a sheltered life and is not prepared for how harsh the world can be. She is a symbol of purity and goodness in the novel. This makes her rape and prostitution all the more heartbreaking.

As the quote above indicates, Jurgis's age and good health also make him an innocent. He is very strong and able to work very hard at first. He can't imagine that it will ever be any other way, or that later on he will have trouble finding work as his body breaks down under all his relentless labor.