How many months old can a baby suitable to sit on bumper chair ?

Hi do you mean bumbo seat? You can place them in it when they are able to hold necks upright and sit assisted or sit unassisted for a short while. I put my gal in around 5.5mo. She was able to sit well assisted and when unassisted last a few minutes. But pls be careful as bumbo seat can be very dangerous, they may topple over. Try to put the seat flat on the ground. When my gal gain the strength to sit on her own and was attempting to kick herself off the ground I stopped letting her sit in it.