There are several conversion tables from cat age to human age. I picked one from them, because it has a clear basis from their researches. That is:

They have studied the growth of cats for many years. And that introduced this conversion table. They divide cats life time into 6 stages. And they corresponded the growth stage with the stage of human growth. 

Acording to that, a cat grows to about 10 years old in six months of calendar age, and 15 y/o in 12 months, 24 y/o in 24 months. After that, it grows 4 y/o in on calendar year. So the table is following;


CalendarAge of Cat
1 months1
2 months2
3 months4
4 months8
6 months10
7 months12
12 months15
18 months21
24 months24
3 years28
4 years32
5 years36
6 years40
7 years44
8 years48
9 years52
10 years56
11 years60
12 years64
13 years68
14 years72
15 years76
16 years80
17 years84
18 years88
19 years92
20 years96

I ploted it in a graph of first 24 months.


Also in 20 years.

This is a table from ICC themselves.

There are 6 life stages for cats:
Kitten – 0-6 months
a period when the young cat is growing rapidly and is usually not quite sexually mature

Junior – 6 months-2 years
during this time the cat reaches full size and learns about life and how to survive it

Prime – 3-6 years
the cat is mature physically and behaviourally, and is still usually healthy and active, looking sleek and shiny and making the best of life

Mature – 7-10 years
the cat is what we call 'Mature', equivalent to humans in their mid-40s to mid-50s

Senior - 11-14 years
takes the cat up to the equivalent of about 70 human years

Geriatric - 15 years and over
many cats do reach this stage, some not showing any signs of being geriatric at all!