My males reach a big enough size to not fit through the bars of my Ferret Nation 142 at around 6-8 months. I don't know if I'd use chicken wire to modify the cage. Most people on the forum who have that cage use hardware cloth and there's even a tutorial stickied at the top of this section.
6-8
months, really?

Usually it's about 2-3 months old for my boys, and about 3-5 months old for my girls.