During which stage of life does the LIP undergo its greatest growth in length?

Correct answer: C. Adolescence

• A. In old age the lip LENGTHENS slightly and thins, but through loss of tone and elastic recoil rather than growth — this is ageing, not growth. Clinically it reduces maxillary incisal display and increases mandibular incisal display.
• B. In middle adulthood the lip is essentially stable in length; the changes at this stage are in tone and vermilion fullness rather than length.
• C. CORRECT. Lip length increases most during ADOLESCENCE, tracking the pubertal growth spurt of the craniofacial soft tissues. Upper lip length continues to increase into the late teens, and this is why smile design and incisal display planning in a growing patient can be invalidated a few years later — the same incisal edge position that gave an ideal display at 12 may be over-covered by 18.
• D. Infancy is a period of very rapid overall craniofacial growth, but the specific increase in lip LENGTH peaks with the adolescent growth spurt, not in the first two years.

Note: High-yield esthetic corollary: maxillary incisal display at rest falls from roughly 3–4 mm in the young adult to near zero or negative in the elderly, driven by the combination of lip lengthening and incisal wear.