The 50-50-90 rule: Anytime you have a 50-50 chance of getting something right, there's a 90% probability you'll get it wrong.
Question #79923 posted on 11/16/2014 3:26 p.m.
Q:

Dear 100 Hour Board,

How is it that pet food companies consider my cat a senior at age 7, when cats live an average of 15 years? So he's supposed to be a senior for half his life?

-Not a crazy cat lady

A:

Dear Katniss,

As senior cat foods are generally more expensive than normal cat foods, it advantageous for companies if you are persuaded to consider your cat a senior sooner. 

Skepticism aside, this veterinary medicine website does suggest "as a general rule of thumb, a cat who is 8 to 10 years old and older should be considered middle to senior aged, and a consultation with your vet is in order to determine the best health care maintenance program for your cat as s/he ages." Of course, this depends from cat to cat.

--Ardilla Feroz