The best way to estimate a cats age is like this:
In the first year of cats growth they age rapidly and become about human age 15.
After that a rough guide is to add 4 years for each human year.

Cats ages dont seem to be an exact science, so this is only a guide. An average lifespan of a cat is normally between 15 and 19 human years, so the following should be sufficient. I think you can work it out from there on in if your pal is older than 20!

HUMAN/ CAT
I

1. 15
2. 19
3. 23
4. 27
5. 31
6. 35
7. 39
8. 43
9. 47
10. 51
11. 55
12. 59
13. 63
14. 67
15. 71
16. 75
17. 79
18. 83
19. 87
20. 91