I don't think any age is too young. Theist start tryignto brainwash kids as young as they can get em.

Being open and honest with your kids from a young age, will mean you probably wont' have as many problems with them when they become teens... that is if they know they can always be honest and open with you too.

As soon as you can teach them to reason.

Always be honest about your (lack of / un) beliefs and your reasons for them, while making it clear that your children's beliefs are for them to determine ultimately. Ask them what THEY think and why. Help them reason through it.

This isn't a question like the one surrounding Santa Claus, where there's an in-joke that adults have on their kids but everyone understands they're not going to ultimately follow through on it. So to pretend to give answers acceptable to believers is not helpful in my view.

It also needs to be age appropriate and to not go beyond the child's actual level of interest. A passing question about, say, what heaven is like, doesn't require that you launch a theological exposition or a logical takedown. My impulse would be to say, well, heaven is something a lot of people believe in, and those people often say it is like x. Why do you think this is such a popular belief that people have? And let them explore that with you. Maybe also mention some alternate concepts of the afterlife, such as the eastern cyclic afterlife concepts. Don't say much about your beliefs, just guide their reasoning -- already here you have pointed out that it's just a belief, and while many people share such beliefs, others do not or have significantly different views. Already it's something that has entered the realm of open discourse and has to compete in the marketplace of ideas, not some sacrosanct thing that just has to be assumed and embraced.

Whenever they ask and you can explain that other people think differently.

The sooner the better. You owe them the truth. I don't get it. What's the problem?.

When the 6 year old daughter of my best friend asks me things about god (the girl goes to Sunday school and loves it) my policy is to begin sentences with 'Some people believe...'. This way I am not telling lies. I hope this helps. If she ever asks me what I think I will tell her directly.