Babies STTN when they are ready. For some, that's at a few months old, For others, a few years. DS started at 6 months, just randomly one night. He had been increasing his milk intake during the day, and self-weaned the night feedings. He usually sleeps from 5:30 pm-6 am, with some variance.
This! My little guy started sleeping through the night (9 PM - 5 AM) at 2 months, but we were very lucky. There are mom's on my birth month board (my guy is 9 months now) whose little ones still don't sleep through the night. They will do it when they are ready. I agree with PP though, get on a regular sleep/feeding schedule if you can, sometimes this can help :)