K.A.
I loved our sit and stand when my boys were younger. They are 3 years apart. We started using one when my youngest boy was able to sit up more, so 5 months I'd say maybe. We used one of those inserts the first few months to give him a little more "coosh" until he got bigger. We never used an infant carseat, instead opting for a convertible carseat that stayed in the car at all times but a sit and stand does have the attachment bar to put the infant carseat bubble on if you use that.