Final answer:
The example best illustrates the concept of chronological age, which is the actual number of years a person has lived, differentiating it from biological, social, or psychological ages.
Step-by-step explanation:
The example of two of Dr. Brown's patients who are in identical health with the same medical conditions but are of different ages highlights the concept of chronological age. Chronological age is simply the number of years someone has lived, as opposed to biological, social, or psychological ages which take into account factors such as physical condition, societal roles, and mental state, respectively. In this case, despite their shared health status, there is a 30-year difference in their chronological ages, emphasizing that there can be a significant variation in ages without necessarily reflecting the physical or mental well-being of the individuals.