Some experts encourage you to wait until your baby is six months old because breast milk and formula are still easiest for your baby to digest, providing all the calories and nutrients she needs and because the chances of developing allergies are greatest during infancy, so feeding your baby a diet of breast milk or formula for as long as possible reduces the risk of introducing allergens. Others say you can start solids at 4 months because that is about the time when your baby is physiologically ready to accept those foods and because some studies show that babies who are introduced to different kinds of foods at an earlier age are less likely to be picky eaters later.
It depends on you, for me I would prefer when my baby is 6 months old.