Final answer:
Primary keratinisation defects are considered unusual or rare in childhood, with the risk of these disorders increasing with age due to the potential for more errors in skin cell regeneration.
Step-by-step explanation:
Primary keratinisation defects are unusual or rare in childhood. The chance of developing primary keratinisation disorders increases with age. This heightened risk stems from the fact that as people get older, there is an increased likelihood of errors occurring in the mitotic cells responsible for skin cell regeneration, which can lead to defects in keratinisation.
In contrast, the development of the frontal lobes, while related to the development of the body and brain, happens at a different life stage. According to other information provided, the frontal lobes become fully developed at the end of adolescence, signaling significant cognitive and psychological growth that coincides with this phase of human development.