6 months. But you just don't let them sit and cry. You let them cry, then soothe, cry, soothe until they fall asleep. I would read about the Ferber method.
I disagree with this age. Not all experts agree either. You really have to read your child and figure out what works for them AND the parents.
We started DD when she was 4.5 months and it has been working well for us.
We do go in and check on her. But she tyically falls asleep within 10-15 minutes. If she is crying hard beyond 15 minutes then I go in and nurse her and she'll fall asleep after. But that is party because I have problems with supply and if I nursed her for her before-bed meal, I can never be 100% certain that she was able to get enough. More often than not, however, she falls asleep. And she STTN more nights too.