Formula fed babies should not eat earlier than 4 months or later than 6 month.
In both mentioned cases, we should be sure than the baby has the signs of being able to be fed solid food.
Next, always introduce one type of food at a time so if your baby has an allergy, you will know what he's allergic to.
Remember honey (risk of botulism), citrus (acidic) cows milk should not be started before the age of 1 year.
The decision to start solid food depends on the child's weight, if he is breastfed or not, his general condition.
Your pediatrician can help you decide when and how to start solid food