The suckling reflex and infantile swallow normally disappear by the age of:

Tongue–to–lower lip apposition is so common in infants that this  posture is usually adopted at rest, and it is frequently possible to gently move the infant’s lip and note that the tongue tip moves with it, almost as if the two were glued together. As the infant matures, there is increasing activation of the elevator muscles of the mandible as the child swallows. As semisolid and eventually solid foods are added to the diet, it is necessary for the child to use the tongue in a more complex way to gather up a bolus, position it along the middle of the tongue, and transport  it posteriorly. Reference: Contemporary orthodontics, Proffit 6th ed page no 64