Answer: C. 18 years old

Explanation: The legal age for entering into most contracts, including employment contracts, is generally 18 years old. This is because, at this age, a person is considered to have full legal capacity to make binding commitments.

• Option A (16 years old) and Option B (17 years old) are below the age of majority and typically indicate limited capacity to enter into contracts without additional safeguards or parental consent.
• Option C (18 years old) is the standard legal age of majority, where an individual is considered to have full contractual capacity.
• Option D (19 years old) and Option E (21 years old) are above the required age but are not the minimum required.

Therefore, the correct answer is 18 years old, which is the age at which a person can independently enter into a contract of employment.