The correct answer is C. 4 years.
In the field of pediatric dentistry and orthodontics, thumb sucking is classified as a physiological sucking habit during infancy and early childhood.
Clinical Rationale
Physiological Phase (Birth to 3–4 years): Sucking is a natural reflex in infants (nutritive and non-nutritive).
Most children naturally decrease this behavior as they develop other coping mechanisms and social awareness. The "Threshold" Age: While many children stop by age 2, the consensus in orthodontic literature (including Graber and Moyers) is that the habit is considered physiologically normal up to age 4.
The Significance of Permanent Eruption: The primary concern for an orthodontist is the transition from primary to mixed dentition. If the habit is discontinued before the eruption of the permanent incisors (typically around ages 5 or 6), the dentofacial changes—such as an anterior open bite or flared maxillary incisors—are often self-correcting.
Comparison of Age Milestones
| Age | Clinical Significance |
| 0–2 Years | Normal sucking reflex; no intervention required. |
| 2–4 Years | Habit usually wanes; considered normal but should be monitored. |
| 4 Years + | Termed a "persistent habit." Intervention (behavioral or myofunctional) is often considered. |
| 6 Years + | High risk of permanent malocclusion (Posterior crossbite, increased overjet, and Open bite). |
Scientific Considerations for MDS Residents
As you are in your third year of MDS, you'll recall that the magnitude of the dental deformity is a result of the Trident of Habits:
Duration: How many hours a day (the "6-hour threshold" is critical for tooth movement).
Frequency: How many times a day the thumb is inserted.
Intensity: The amount of force applied (negative intraoral pressure vs. physical leverage).
Note: If the habit persists beyond age 4, it is no longer considered purely "physiological" and begins to transition into a "psychological" or "learned" habit that requires professional guidance.
