**Core Concept**
The rate of mixed whole stimulated saliva plays a crucial role in oral health, and its variation with age is an important aspect of understanding oral physiology. The flow rate of saliva is influenced by the parasympathetic nervous system, which controls the secretion of saliva by the salivary glands.

**Why the Correct Answer is Right**
According to Crossner, the rate of mixed whole stimulated saliva increases with age in the elderly population, typically above the age of 60. This increase in saliva flow rate is attributed to the higher parasympathetic tone and the increased sensitivity of the salivary glands to stimuli. The parasympathetic nervous system, mediated by the vagus nerve, stimulates the submandibular and sublingual glands to secrete more saliva, which helps to compensate for the decreased saliva production in the elderly.

**Why Each Wrong Option is Incorrect**
**Option A:** This age group does not show a significant increase in saliva flow rate. The rate of mixed whole stimulated saliva remains relatively stable in young adults, and there is no notable increase until the elderly age group.
**Option B:** This age group might show some variation in saliva flow rate, but it is not the age group where the rate of mixed whole stimulated saliva consistently increases with age. The pre-elderly age group may exhibit normal or decreased saliva flow rates.
**Option C:** This age group is too young to exhibit the significant increase in saliva flow rate seen in the elderly. The rate of mixed whole stimulated saliva remains relatively stable in children and adolescents.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the elderly population may exhibit a higher risk of oral health issues due to decreased saliva production, making the increase in saliva flow rate in this age group a crucial adaptation to maintain oral health.

**Correct Answer:** C.