It's totally child dependent and I thought sleep training sounded horrible in principle, but I did CIO with my DD at 11 months, up till that point she had been a good sleeper BUT we were cuddling/feeding to sleep, if she woke in the night she would go back to sleep after a quick cuddle/feed etc.
She got to 11 months and bam, she just didn't want to be put down anymore, would wake 45 minutes after finally falling asleep, woke every 2 hours at least, took ages to fall back to sleep, would not settle if my DH went to her, she would scream and cry for me and settle as soon as I walked in, I spent hours in the chair with her waiting for her to fall asleep.
There was nothing wrong with her, I think she was just used to being cuddled to sleep and wanted that every time she woke up, and she had learned that if she cried I'd go to her as I always had, but I was putting her down at 8pm, working at home in the evening till 2am, going to sleep then up at 8am, so when she started waking up every 1-2 hours, instead of once a night I was on the verge of breakdown. One night I had an hours sleep, it was beyond ridiculous and I was exhausted, she was exhausted from the broken sleep too.
I decided to do CIO, because the ferber method I thought would be confusing, the first night she cried for 15 minutes, then went to sleep and sleep through the rest of the night! After 4 nights there was no more crying, she just started going to sleep as soon as I put her down, and was then sleeping 12 hours every night uninterrupted, I honestly couldn't believe it, after two weeks she was happy to go to bed, no tears, no sadness, just kisses, cuddles and goodnights.
My personal opinion is that she needed to learn how to self settle, and once she had that she was happier, more rested child for it, and I was happier getting a full nights sleep again, I think there is an optimum time to do it and only you as a parent know when they are ready to learn, and if it is the right solution for them.