8 weeks Most dogs will take to a harness well, with few exceptions. Puppies can be taught to wear a harness practically from day one as well, as you'll get your pup at a minimum of 8 weeks of age. Give them a chance to settle in for a few days and then start teaching them.

Unless you're on a long hike or overnight camping trip, there's no reason to keep your dog in a harness all day. Harness should be worn outside, but not inside the home. Especially for short-haired dogs, wearing a harness 24/7 can cause friction and spots (mostly on the back of the front legs). Don't put your dog in a harness all day unless you have a reason to. A harness are a practical piece of equipment, not an accessory to be worn for long periods of time.