As long as the baby can sit up on their own unassisted and the baby is eating solid foods then the baby is ready to sit in a highchair.
The baby needs to be able to support themselves while sitting in the highchair and also make sure to strap them in the highchair and make sure the baby is supervised to avoid any choking or other hazards.
Never leave a baby or child unattended while in the highchair.
Usually around 4 to 6 months of age most babies are ready and able to sit in a highchair.
My babies started using a highchair when they were 4 months of age which made it easier for me to feed them.
Some babies are not ready to sit in a highchair until 6 months to 7 months of age but as long as your baby is sitting up unassisted then they can sit in a high chair.