Final answer:
The expression that would be true for indicating that a person is a teenager, given an integer variable age, is '13 ≤ age ≤ 19'.
Step-by-step explanation:
The expression that will determine if someone is a teenager, given that age is an int variable, is 13 ≤ age ≤ 19. This means the age must be greater than or equal to 13 and less than or equal to 19. The age range for a teenager is because individuals are typically referred to as teenagers from the time they turn 13 until they turn 20, at which point they are no longer referred to as teens.