I've always heard/read that you can begin CIO at 6 months. Before that, they just don't understand. They can learn that adults will not help them when they need help, and can disassociate and have social problems later. Of course, those are severe cases, not just from one or two episodes of being left to cry too long.
And as a mother of three, I'll tell you that CIO works with SOME KIDS. It worked like a charm with my first. I knew that if he fussed for longer than 5 minutes, I needed to go get him up, because something was wrong. I thought I was an awesome mom, because I had it all figured out. Then baby #2 came along and threw all that out the window. He would cry for 45 minutes at age 8 months and never calm down. He just worked himself into a tizzy. I learned that what he needed was to cry for a minute on his own, then I needed to go comfort him and lay him down again, and then he'd sleep. He's still that way--if he's crying, he needs some time to feel sad, and then he needs someone to go comfort him for a minute before he can stop.
Baby #3 is NOT a cuddler or a CIOer. Luckily, he's a blankie/pacifier baby. I swaddled him for a long time, maybe 7 or 8 months? Even in the summer, although I had to use just some lightweight cotton fabric instead of a blanket. He just needed it. He now goes to sleep soooooo easily. I lay him down, hand him his paci, hand him his blankie, and wind up his stuffed musical dog, and we generally don't hear from him again.
So you're going to have to figure out what's going to work with THIS baby. It really matters more about what he needs and less about what you think will work.
That's what I've learned from having three kids!