sambo - That 6-month figure is not gospel. DD's paediatrician here in France told us to sleep-train her at 4 months. She said a 4 month baby does not need to without a feed but wakes up out of habit. So that habit needs to be changed into the habit of feeding more in the day and not feeding in the night. In order to do this, she told us to stop night feeds.
There are different ways to sleep train. There is "controlled crying" ("CC") where parents leave baby to cry but go in at increasing intervals (1 minute, 3 minutes, 5 minutes, etc).
We didn't leave DD to cry, but stopped night feeds at once. When she woke up in the night, DH went to her and tried to put her back to sleep any way possible - rocked her, sang to her, patted her, carried her around the room, etc. First night, this took many hours. Second night, it took about an hour in total. Third night, DD slept through from 8 PM to 8 AM.
Hand on heart, it was the single best advice I received from anyone, about anything. DD was a much happier baby once she started to sleep well, and I was a much better mum when rested.