Translating a cat's age in real years into the human equivalent often seems to make it easier to understand quite where they are in their life cycle when we're talking about age-related health problems.
Everyone has been younger than they are now, and most of us know at least some older people and the health issues they're going through. So when we think of Mr Tibbles as being the cat equivalent of a 15 year old boy or a 45 or 75 year old man rather than his real 1, 7 or 15 years, his health and vitality issues can make a bit more sense.
There's far too much nonsense about each cat year being 7 human years or variations on that theme - if it were true, the average life expectancy for a cat in the UK - measured in human equivalent years - would be just over 100.
I suggest you use this: it's from International Cat Care