Thumb sucking is considered physiologically normal up to 3½ to 4 years because during this period, the habit usually helps with self-soothing and is unlikely to cause long-term dental or skeletal problems. After this age, prolonged thumb sucking can lead to malocclusion such as anterior open bite, posterior crossbite, and altered palatal development. Early intervention after 4 years is recommended to prevent these orthodontic complications. Therefore, the habit is generally accepted as normal within this developmental window but becomes pathologic if it persists beyond it.

Reference: Nelson Textbook of Pediatrics, 21st Edition.