Typically, infant medication is manufactured with children under two years of age in mind. When your child closes in on 24 months, you should think about changing from the infant meds to children's meds. Weight. Even though the difference between infant and child is technically benchmarked by age, when it comes to medication dosage, it's more accurate to go by weight. In most cases, when your child hits the 25 pound mark, it's time to graduate to child's medication.