Any time and any age that you let them range they are in danger of predators, even if you are with them. I've seen a few people post that a fox, bobcat, or hawk got their chickens within 15 feet or less of where they were standing. Your presence is a huge deterrent but there are no guarantees.
I let my first batch of chicks free range at five weeks out in the country. We saw foxes, coyotes, and hawks but never had serious predator losses for three years. Then I had two back-to-back dog attacks, different dogs, when people abandoned dogs out in the country for the good life. So I put them in electric netting. Electric netting does not stop flying predators but I haven't had a problem there. It did stop all ground predators.
When i was growing up, Dad free ranged a flock of 25 to 30 chickens. Until after I left home for college we had only two predator attacks, a fox and a dog. Both were shot. Some people are able to free range for years without issues. However, others get wiped out as soon as they try it. This is just as likely in the city as in the country. They are at risk every time you let them out. I can't tell you how much risk.