Yeah when they can hold their head up, and most prams recommend lying flat until 3 months. So somewhere after 3 months when they're strong enough not to have their head rolling around. My first was fine even at 2 months, he was this strong, giant of a baby who held his head up from day 2 after birth and hated lying down. My second was comfortable in there from 6 months or so.
I agree with the above. As long as they are holding themselves up then it should be fine :)
I would say Depends of your baby. If he or she knows how to hold there head up and balance. My neice was 5 months when I let her Sit in the stroller without car seat.