Conversation so far:
where did the concept of dog years come from
No one knows where the dog years rule came from but it is a popular myth
when was the first mention of the mythology of dog years
A Judgment Day calculation inscribed at Westminster Abbey in 1268 declared a ratio of 9 to 1, meaning that humans live to age 81, and dogs to age 9.
when did veterinarians espouse the 7 year rule
At some point in the 1950s, a rhetoric emerged that humans generally live 70 years, and dogs 10. From there, veterinarians likely espoused the seven year rule as truth.

Question: was the rule a marketing ploy
Self-contained rewrite: was the 7 year rule a marketing ploy by veterinarians