DS1 was 11 weeks and just did it, Dd was 20 weeks and just did it too. No techniques, no training.
Ds2 was 2 yesterday and still isn't very reliable. He can go 7pm - 5:30am but regularly wakes at about 3am. Until 18mo he woke up to 6 times a night, more on a bad night and we tried every sleep training going bar just leaving him completely alone to scream himself to sleep which I feel is barbaric. Nothing worked and he has gradulayy improved by himself.
I have since come to the conclusion that some babies/toddlers/young children are just crap sleepers and all you can do is go with it and find a coping strategy as best you can. It's entirely down to luck and nothing else.
There are always smug mums who express absolute shock and disbelief at the fact that your child hasn't been sleeping through since day dot and give lots of 'helpful' tips on how to train them to do it. I usually want to punch them in the head, but instead I smile sweetly and hope like mad that their luck runs out with their next child and they get a sleeper from hell. At least then they might realise that children who don;t sleep are not the fault of the mother parents and sometimes there is bugger all you can do to make them sleep until they are ready.