My girl is 2 months and I have been using Ergo carrier few times. My in-laws are saying she's too young for it. But she fell asleep when she's in it. Plus baby carrier is more convenient than pram.
there's a type of ergo baby carrier that comes with an infant support as an extra piece - you can discard or keep that once baby is older. so if you're worried about putting your baby into the carrier when he/she's only about 2,3 months old, the infant support helps elevate baby upwards such that he/she will be resting against your chest. you can also always consult the staff at mothercare (or wherever you're getting your carrier), and it does help to bring baby along when you try the carrier out
you can babywear from newborn. depending on the type of carrier you use. also size of baby and mother... there are many to choose from. brands like ergo, manduca, baby bjorn have buckles which are quick and easy. otherwise you can try traditional cloth carrying which takes much more practice.