I'll just speak from my experience. I started my two kids at around 8 years old, they're 11 and 13 now. We have early youth firearm deer season here in Illinois. It's a great deal since weather is generally mild. It started with low recoil guns in blinds with snacks and things. Even since the beginning, I always made the hunts just a little longer than necessary. If deer movement is the last 2 hours of the day, we'd go out an hour before that. I wanted to slowly ease them into being comfortable with seeing nothing, being alone with their thoughts, and being patient. Now they can both do a 4-5 hour sit like it's nothing.
After some success, I started taking them on more sits. Archery(crossbow for them), treestand hunts, late firearm, hunts with extremely cold weather. We'd still stick to the blinds with heaters but they'd still have to deal with the elements. My daughter at 10 had to help drag out her buck when it was -10 and blowing.
I then started getting them into other species. My son's antelope hunt this year at age 11 was 95 degrees and his buck was a mile hike each way. Absolutely brutal packing it back out. Him and I also did a DIY Aoudad hunt in TX. He killed one at the top of a decent mountain and it was 100 degrees by time we packed it out at noon. He was drenched in sweat and full of cactus thorns. It sucked even for me and I've dealt with some bad ones. He handled it like a pro.
I took my daughter for late cow elk two seasons back, also at age 11. Opening morning was -10 and snow. Just lousy weather. She was miserable but still hiked 5-7 miles a day and killed a cow on day 3. We went back this year in November and the weather sucked again. Brutal cold. She almost cracked on that one. Didn't kill a elk but toughed it out. After the fact, she still talks about those hunts with fondness and wants to do it again.
In a world of comfort it's necessary to create discomfort in order for a kid to grow. Letting them sit in front of a phone or X-box all day borders on negligence in my opinion. I've progressively increased the suck factor over time on our hunts. Now both of them can hack it pretty well. They complain a whole lot less than some grown men I've hunted with. I tell my wife all the time that I'm not raising lazy or soft kids, and so far it seems to be going ok. You'll have moments when you look at them and know they're not happy or uncomfortable, but as long as you know they're safe and it's not too extreme, that they're growing from it.