Around 6 months of age is when most babies can begin sitting in the shopping cart seat.
A baby can sit in the shopping cart and shopping cart seat when the baby is old enough to sit up on his/her own without assistance.
Like I mentioned above this is usually around 6 months of age but can be at 7 or 8 months of age as it all depends on the baby.
If your baby can sit up on it's without any assistance then the baby can sit in the shopping cart seat and you should make sure you buckle the baby into the shopping cart seat using the provided strap.
Always watch your baby while in the shopping cart seat as well because it's very easy for the baby to get hurt or someone could easily abduct the baby in the shopping cart seat when you turn around.
I've always kept a hand on the babies shirt when I had to look away from the baby to get something off the shelf and it's a good idea if possible to have someone such as the other parent around with you when you take your baby shopping so they can keep an eye on the baby.