Step 1: Understanding the Concept:
In comparative English sentences, comparisons must be drawn between grammatically parallel and equivalent entities.
Step 2: Detailed Explanation:
The original sentence illogically compares an abstract measurement ('average life span') directly to a person ('a member of the aristocracy').
To maintain logical parallelism, the singular demonstrative pronoun 'that of' must be used to refer back to 'the average life span' ('12 years less than that of a member of the aristocracy').
Step 3: Final Answer:
Therefore, 'That of a member of the aristocracy' is the grammatically correct replacement.