Final answer:
The age criterion being used when someone turns 40 years of age on August 21st is Chronological age.
Step-by-step explanation:
The age criterion that is being used when we say that someone turns 40 years of age on August 21st is Chronological age. Chronological age refers to the actual number of years a person has been alive. It is the most commonly used criterion to determine a person's age.