For most SSCs (barring those that have bases that can cinch like the pikkolo) once baby is big enough to comfortably straddle the width of the carrier. You can also take a small blanket and fold it up to make a little seat for LO if they aren't quite big enough but don't want to be legs in (or if they are pushing off on their legs when froggied). This will boost them up a bit in the carrier and allow them to be in the sit squat position without over-spreading their legs (since they won't be right on the base of the carrier their legs will still be able to bend at the knee).
Re: Soft structured carrier
Breastfeeding Counselor with Breastfeeding USA
Babywearing Guide ** Newborn Carriers
Cloth Diaper Guide
Safe Bed Sharing Info