How old can chicks be before i turn the light off in the brooder?

That’s a little hard to answer. Not all coops and brooders are built the same. Some are more drafty than others on the floor where the chicks will be sleeping. Having bedding for them to snuggle down in will keep them warmer. An elevated coop will cool off a lot faster than a coop on the ground where the soil acts as a thermal mass. Larger numbers of chicks like you have will help keep each other warm when they snuggle together.

Not all chicks feather out the same. Most normal chickens will fully feather by 4 to maybe 5 weeks, but some more decorative chicks may feather out slower.

It also makes a difference in how they are kept. If the entire coop or brooder is kept at tropical temperatures they are going to feather out and acclimate slower than if they are exposed to colder temperatures. A lot of us heat only one small area of the brooder or coop and let the rest cool off as it will. The chicks soon play all over the brooder, even if the temperatures in part of it are pretty chilly, going back to the heat to warm up when they need to. Some people would be surprised how much time they spend in the cooler parts of the brooder. This is a lot like a broody hen does it. She does not heat up the entire world, just provides a warm place for the chicks to go when they need it.

I’ve put 5 week olds in my unheated elevated grow-out coop when the overnight lows hit the mid 40’s Fahrenheit, but that grow-out coop had great draft protection. The overnight lows hit the mid 20’s F just three nights later and they were fine. But they were well acclimated by me only heating a small part of their brooder and letting the rest really cool off.

In the heat of summer, much warmer temperatures than you are having, I’ve turned the daytime heat off at 2 days and the overnight heat off at 5 days. I watch the chicks and see what they are telling me. Those were getting as far from the heat as they could, even at night. Again in my summer temperatures, I’ve seen broodies take their chicks to the roosts at 2 weeks where not all could fit under her. One slept on a 2x4 brace on the side of the coop wall, not even close to the broody’s warmth.

I don’t know what your coop looks like, what breeds you have, or how well they are acclimated so I’m reluctant to give a magic number for you. I don’t know what your daytime temperatures are. What I’d suggest is to only heat one small area and let the rest cool off so they can acclimate. If you are around during the day, turn off all heat while you can occasionally observe them and see how they react. They may not need any heat during the day. Give them a chance to get acclimated to cooler temperatures. Yours might be ready to go without any nighttime heat in another week or to be extremely safe, wait until they are five weeks.

In the meantime, cover those wires with something so they can’t peck them.