The short answer is; when they were about 5 years old.
My boys had always worn harnesses and they only felt safe when they were wearing them so as they grew they would ask to wear them even though it wasn't necessary. My boys decided when to stop wearing their harnesses and because I trusted their behaviour, I left the decision up to them.
But in reality, there is a transition period that happens between using the harness every day with your child and leaving the harness at home for good.
- initially, when you first use my Child Harness with your toddler they discover that they can run only as far as the end of the lead. Before they were able to run as far at they wanted (which is why you ordered the harness!!), now there is a limit to their range. But they soon figure out that the harness gives them an immense amount of freedom and control over one aspect of their lives: they decide when they walk or ride and they are no longer strapped into their stroller against their wishes.
- next, your child will quickly associate the harness with outings. Putting it on is just like putting on their shoes or their jacket. It's part of the routine and they don't question it. The exciting thing is the outing and that's all they care about. For me, this phase lasted until the boys were about 4, 4 1/2.
- as your child grows and matures, you'll have more confidence in their ability to stay close to you during your outings, especially if those outings are to familiar haunts like the grocery store or the mall during a quiet time of the week. You're not dealing with an unpredictable 2 year old any more but you're also not dealing with someone who listens to you ALL the time. The harness is a good precaution in case there is a disagreement between you and your child and your obstinate 3 1/2 year old decides to take off on you.
- as they mature even more, disagreements between you and your child become discussions of reason and compromise. You know their flight risk is close to zero so if they ask not to wear the harness, you agree. But you take it with you "just in case". My Child Harness is not bulky like an animal backpack harness, it bundles into it's own stuff sack so it's easy to tote along.
- transitioning into the phase of not using the harness at all is a signal to your child that they are mature and trustworthy and you take great pride in their behaviour to act responsibly when you are out together. You'll likely find yourself carrying the harness with you needlessly for months, then decide to leave it at home that one time when disaster strikes. That happened to me and from then on I always carried their harnesses with me, no matter where we were going and no matter how long.