We let our older three CIO at different ages, anywhere between 9 mos and a year. My kids are the BEST sleepers, now! If you ask them how traumatic it was for them to CIO, they'll ask, "What?" LOL! I know it's a controversial topic, but it worked for us.
I think the decision was made when the crying became obvious that they wanted to play, rather than eat. It took a night or two, but everyone slept better for it!!
That being said, I have a "straggler" (surprise baby after 7 years) :). She was so different than the other three kids, so I had to do things differently with her. She fought naps from the day she was born, almost! She would also cry herself into a fit - way more so than the other kids did at her age. So, before she was mobile, I would lay down with her in my bed. She wanted to sleep, but would cry and cry - fighting it tooth and nail. She would eventually fall asleep - so basically crying it out, but with me right next to her. She couldn't roll over yet, or crawl, so she had to lie next to me and cry herself to sleep. This method worked so well, because, when it came time to lay her down for naps or at night, she no longer fought it. We never had to do the CIO at 9 mos or a year, because all we had to do was lay her in her crib, and she put herself to sleep. I wish I had done this with the other kids (not that I had the option, when I had toddlers, as well as the baby). It took less than a week of laying with her during naps to get her to stop fighting them. It relieved some of the guilt I felt by the "normal" CIO method, since I was next to her, I didn't feel like I was abandoning her. Of course, even though I had those feelings at the time, when my baby was crying and crying, etc. I still knew it was the best thing for them and for me. I never regretted using the CIO method, at all, once they learned to fall asleep on their own.