At 18 months old, Bob would be in the sensorimotor stage of cognitive development, according to Jean Piaget's theory of cognitive development. This stage typically spans from birth to about 2 years of age. During this period, infants and toddlers explore the world through their senses and motor actions. They begin to develop object permanence (the understanding that objects continue to exist even when they are out of sight), and they engage in increasingly complex forms of play and problem-solving as they grow and develop their cognitive abilities.