I have the Baby Bjorn and their website said you can use it from birth, but baby must weight 8lbs or more. My baby weighed over 8 lbs at birth, so we used ours within the first two weeks or so (I had a c-section, so it was that long before I felt up to carrying her around.)
We used the ERGO baby carrier from the first week. My LO was 6 lbs when he was born. We bought the infant insert. He loves it. He falls to sleep right away when in it.
Re: Baby Carriers
It's all based on weight...not age. Most carriers say 8lbs, but some can be used with smaller babies.
I do advise against the bjorn as it puts the weight of your baby on his/her crotch instead of on their butt like an ergo would.
I've been carrying my LO in a mei tai or Moby since she was about 7.5 lbs.
I have the baby k'tan, and it says min 8 lbs. We used it earlier than that, but just for a few mins around the house.
My guess is it depends on which one you have, exactly.