Even older chickens can fly a lot better than many people realize. Whether they do or not depends on their motivation. I keep mine in 4’ high electric netting. They could easily fly over that but they don't with one exception. They have lots of room and obviously don't feel a need to fly out.
I generally integrate new chicks at 8 weeks of age. Occasionally a young one will get trapped near the fence by an adult and in its efforts to get away, it will fly over the fence. With 15 to 20 chicks being integrated this happens maybe once or rarely twice a week and it generally stops around 12 to 14 weeks of age.
In the past I had hens get out of a 5' high run. Their motivation was trying to get away from an amorous rooster and they were trapped in a corner.
Other than motivation, there is another possible reason they might escape. If the top of your garden fence is solid so it looks like they could perch on it, they may fly up there just to perch. They enjoy perching tremendously. Once up there, who knows which side they might decide to hop down on?
I don't know what your garden enclosure looks like, how big your garden is, or what motivation they may have to get out. I'm not going to just blindly say they will be fine, don't worry about it. I will say that many of us keep then inside a 4' enclosure without any real problems. I'd think you'd have a pretty good chance of keeping them in with a 6' high fence. If it has a top rail where they can perch, you are at more risk. But there is only one way to find out what will happen in your specific case.
Good luck!