How do you determine the age of a praying mantis?

As far as I know, you can only determine it to an extent. If they have wings at all (females will have shorter wings, sometimes harder to see, males will generally always extend past the end of their abdomen), they are a full adult, which would indicate anywhere beyond 2 months, depending on species. You can also judge based on size, as they generally double in size each molting. If they are about 1/4 an inch, they are newborn, then they molt each week or so, doubling in size. So if it's about an inch long, they would most likely be 2-4 weeks old.