At 48 I wouldn't consider myself old nor any one a similar age to me! Unfortunately I have noticed changes to my body which I know is related to the degenerative process of ageing!
At 65 you are officially an 'old age pensioner' and many insurance company's give extra benefit's to the over 55's. In the UK you have to undertake a medical exam at the age of 70 to determine your fitness to hold a driving license.
On the other hand, I have often been told life begins at 40 and the other little gem 'You're never too old!'.
I agree with the other answers, it's the younger generations perception that defines old.
In the minds eye of a 48 year old mature man 'old' is now related to as someone having to walk with a stick or some other infirmity like arthritis or having to have constant 24hr care! 20 years ago it was all about the Grey and bald bits, so yes my perception of old has changed in line with my age!
In my opinion I think anyone 'retired' is old!
Was that an ageist comment?