Final answer:
Solid foods can be introduced around six months of age. Start with single grain cereals, followed by pureed fruits and vegetables, and gradually introduce more complex foods.
Step-by-step explanation:
According to developmental milestones, solid foods can be introduced around six months of age. At this stage, infants are able to pick up objects, transfer them from hand to hand, and sit up on their own. As for the order of introducing solid foods, it is generally recommended to start with single grain cereals, such as rice or oatmeal, followed by pureed fruits and vegetables, and then gradually introducing more complex foods like mashed proteins.