I started using it probably around 4 or 5 months old with DS1. Pretty much as soon as he had better head and neck control. But he hated his car seat so it wasn't very fun to leave him in the car seat screaming while we did errands, so we may have done that a bit earlier than we would have otherwise.
It depends on the stroller. We specifically got one that you can use the stroller from birth because we do a lot of walking in our neighbourhood and on paved trails. Not jogging or anything, just smooth road walking. Our stroller adjusts to lie basically flat and has head padding so it's safe for newborns.
If the stroller you have is in a seated position I think you have to wait until the baby can sit up on their own to use it.
I've heard you need to wait until they can sit up unassisted which is anywhere from 4 to 6 mos.
This. I can't wait to be done with the bucket seat in the stroller. DO loves being awake for stroller rides (not a nap helper) so he will love sitting up and looking around.
Re: When to use travel system without car seat?
DS1 - 7/2011, DD 12/2012, DS2 - 4/2014, MMC - 12/2015