Babies can sit in a high chair as early as 4 months but the baby is certainly ready to sit in a high chair at 6 months of age.
As long as the baby can sit up on their own without assistance and is eating solid foods then you can put your baby in a high chair to feed them.
Be sure to keep an eye on the baby in the high chair and strap the baby into the high chair properly to keep them safe.
Never leave the baby unattended in a high chair though because they could be injured etc.