During the puberty period, lip separation at rest is a usual occurrence. In adolescents between age 4 and 20, vertical skeletal and dentoalveolar growth is completed first before vertical lip growth.
If a child has lip incompetence at the age of six, most usually he will experience “self-correction” of the lip incompetence when he reaches the age of 16. In orthodontic diagnosis, lip incompetence is emphasized not only because of its esthetic effect but also because it is associated to the stability of overjet correction. Lip incompetence in younger children is most often related to short lips. In reality however, it is because of incomplete soft tissue growth and is regarded as normal in most adolescents.
Studies show that in girls, the upper lip reaches its maximal thickness at age 14 and remains the same until age 16. Afterwards, lip thinning will occur. On the other hand, boys at age 16 achieve their maximal lip thickness and after that, lip thinning will occur. In both boys and girls, the horizontal thickness of the lower lip has attained its maximal thickness by age 15.