STRAIGHT FROM THE DAILY VET WEBSITE:
Continue to add four years for every year your cat is fortunate enough to live past the age of twenty.

I won’t go into all the details of what is included in the guidelines, but it talks about the importance of wellness exams, nutrition and weight management, diagnostic testing (e.g., blood work, urinalysis, blood pressure checks, and fecal testing), behavior and environmental issues, parasite control, vaccinations, and dental care. 

One interesting tidbit that I took away from reading the report is that "41% of people looking for their lost cats considered them to be indoor-only pets," and that "only about 2% of lost cats ever find their way back from shelters, a major reason being the lack of tag or microchip identification." I had no idea these statistics were this dismal.
    
The guidelines are aimed primarily at veterinarians, but take a look at them yourself if you want to know the reasoning behind your vet’s recommendations or even more importantly, to make sure your cat is getting the care that he or she deserves.

Dr. Jennifer Coates