According to Jean Piaget's theory of infant development, "Cognition", or "the child's new way of understanding the world" is what makes one stage more advanced than another.
Here are the four stages of development according to Jean Piaget:
Sensorimotor (at age 18 - 24 months)
Preoperational stage (24 months to 7 years old)
Concrete Operational stage (ages 7 - 11)
Formal Operational stage (age 11 and beyond)