Both of my kids started young and as noted on here a Pack n Play is a great idea as my kids would sleep in those for all kinds of travel, not just camping. So they were already used to it.

They are now 14(son) & 17(daughter) and are both well in tune w/setting things up and etc. They both lean more towards hammock camping now, as they have gotten older.  As w/most things, I will agree, the sooner the better....

I started my little guy  3 -4 with a kiddie tent to play in while in the house.  When he was around 5 or 6yo, I always had a ratty tent in the backyard for us to sleep in the occasional summer night.  Then I had him sleep in it or forts by himself in another room (den, living room).  We started camping at local state parks around 7 or 8ish twice a year and extended the trip each time e.g. weekend, 4-5 days, 10 days. At each stage we worked on camp basics, pack management, health and safety stuff as age appropriate. Last year (age 10) we road trip car camped with various stops leaving MA and landing in Big Bend NP, including some dispersion camping--3 weeks total.  This last summer was over a month and a half at TRNP, Glacier, YNP, Badlands and more. We have two tents a quick 2person and 4 person for longer stays; he can pitch them both proudly.  He's been starting campfires since last year. For safety reasons, I still check his backpack for longer, higher elevation hikes.

 I would say start them as early as they will tolerate and GEAR THEM PROPERLY (REI, I'm looking at you! :).  If they are uncomfortable (with shoes, packs, temperature regulation), they will learn to hate it fast and it's hard to go back. Invest in leukotape as it is a miracle for blister management..seriously. Get out as often as you can so it becomes the norm.  

Have fun!