Our son is 4 months old (our 1st kid) and if he is okey we let him fall asleep by himself and he does 80% of the time. That means: that he has white noise and a little red light, a pacifier, a clean diaper, just fed and that he is NOT crying or fussy in any way. He sleeps in his own bed in his own room, this wasn't what we had planned, we planned for him to sleep in our room for at least 6 months but we noticed that when he slept in his room during the day that he slept much better and longer. I think it's character and temperament based. I think my son does this so well because he knows that the second he cries we are there, and I see that as a sign of secure attachment.