My 3 month old starting sleeping through the night from when she was 6 weeks old.
Did you just think, "How lucky of you to have such a good baby" ? That's the response I always get. And it annoys me. Yes, this baby actually is really good. She has a calm and chilled-out nature. She's alert and very responsive to human interaction. But that's not why she sleeps so well.
She has been put on a strict routine of sleeping, feeding and wake times since she was 1 week old. This has involved waking her up at times when I just wanted to let her sleep, and putting her down while awake (she, not I) to settle herself to sleep (translation: cry). The long term (ie lifelong) goal is to stabilise her hunger and sleeping habits. The short term effect is that she sleeps through the night from what seems to many to be an impossibly early age.
So while I agree that this wee bundle of joy is indeed a good baby, please note that the more important factor is that she and I have worked very hard for this result. Three kids on, it's still heart wrenching to listen to your baby learn to settle themselves to sleep. It was more than "luck", and more than having a "good baby". Give me some credit man!
PS. I'm always happy to give tips on implementing routines to those who ask :)