It depends what you mean by old as a cats organs do start to deteriorate by about the age of 8, I believe, which is why they are often referred to as senior or geriatric. When Jaffa had his blood tests last year I asked for a senior blood panel and it says on the copy of the results I have that it is a "geriatric" profile
I've been careful not to let Jaffa see that! I've accepted that Jaffa is now a senior cat (he'll be 10 in June) and that I need to pay a bit more attention to preventative health care now, but I don't consider him old. In fact after yesterday's little play with Mosi (they were both racing around chasing each other then had a game of hide and seek with a chair) he's definitely not old! From tables I've seen comparing cats ages with human ages I think 15 is comparable with a 75 yr old human so I'd say mid teens is "old" but not old in a negative way as many 15 year old cats are still very sprightly
edit - the following table is from messybeast. It makes sense to me, although I'm not too sure about the very high ages - it says that age 30 in a cat is equivilent to 136 in a human. 30 may be extrememly rare for a cat but it doesn't seem to be impossible. However, I've never heard of a person reaching 136!
http://www.messybeast.com/towards-end.htmCAT'S AGE EQUIVALENT HUMAN AGE COMMENTS
2-3 months 9-12 months Kittens/humans weaned. Kittens are becoming less dependent on the mother.
4 months 2-3 years Talking/adult communication in children. Under natural conditions, the kitten is fully independent of the mother.
6-12 months 12-15 years Sexual maturity, most females now fertile and able to have young although they may not be fully-grown.
2 years 24 years Could have raised children.
3-6 years 28-40 years Human career-making
6-9 years 40-52 years Middle age spread, menopause for some women.
9-13 years 52-65 years Human menopause and retirement. Most cats are beginning to take things easier.
13-17 years 65-85 years Active but ageing. Signs of senility in some individuals, senses less acute, injuries heal more slowly or incompletely. Internal organs less efficient.
17-19 years 83-92 years Probably frail due to loss of bone density, subcutaneous fat and muscle tone. Skin more fragile. Hearing, sight and mobility affected. Less supple.
19-22 years 92-100 years Amazing.
22+ years 100+ years An exceptional individual
30 years 136 years Several cats have recently attained this age.
34-36 years 152-160 years Official longevity records noted in the Guinness Book of Records.
43 years 188 years Unofficial (unverified) longevity claim; cat was apparently still active and was killed by a train.