Totally depends on your set-up. My brooder is built into my coop. I put them outside on Day 1. They hatch in my house because that’s the only place I can stabilize the incubator temperatures. If I tried to raise chicks in the house my wife would probably leave me. Dust, smell, and noise.
In the brooder in my coop they have good predator protection, protection from the adults, a good draft guard, and I can safely heat one area. I let the rest cool off as it will. That way they can find their own comfort zone. So if you can safely provide heat and keep the older chickens from killing them, you can put them outside anytime you wish.
If your question is when they can go outside without you providing heat, that’s going to vary some. It depends on your weather and a bit on your chicks. Some chicks feather out faster than others. If you have normal chickens they probably are fully feathered around five weeks, but some of those fancy chickens may not feather out as fast. I don’t raise them so I don’t know.
Last summer in our triple digit heat wave with the nights very hot, I turned the daytime heat off in the brooder at 2 days and the night-time heat off at 5 days. In Cornwall I would not recommend that at any time of the year.
I have put 5 week old chicks in an unheated grow-out coop when the overnight lows were in the mid 40’s Fahrenheit, maybe around 8 Celsius. It had good draft protection and they were fine. They were acclimated to the cooler weather some in my brooder because it was big enough and ventilated enough to cool down in the far corners quite a bit. They spent a lot of time in the cooler parts of that brooder, just going back to the heat when they needed to warm up.
I don’t know what your weather is like this time of the year, but I’d think you should be OK sometime after 5 weeks as long as they have decent draft protection. If you are getting down below freezing and have cold wet blustery weather you might want to wait a bit, but you probably don’t need to wait very much. You also have to think about integrating them with your older chickens which I won’t get into. I trust you can handle that.
Good luck!