Not sure why this GP is saying this- seems strange and I would certainly see someone else asap. However, it depends on your general health nd fitness. A young friend'sm mum, who is a few years older than me had two knee replacements that have been a disaster- but she was really NOT well generally beforehand, and suffered from depression and was very unfit- and did not do the necessary physio and exercise to help recovery.
I know many here disagree strongly- but 'my' surgeon said = I can do the ops, but you will have to do the work- you will determine what happens next.
'I wonder whether it's due to wear and tear, osteoporosis or a fall necessitating replacement joints. ' was asked above. Well it depends, each case is different. For me, it was a car accident when I ways 19, which smashed all my right side- and I had a pin to support traction for 4.5 months, in the knee. After 7.5 months in hospital, it took me 2 years to learn to walk properly again and continue recovery- with a right leg 1 inch shorter. So I was always told my knee would need to be replaced, but had to wait until I was 65. The other knee wore too because it had to compensate and do the work for 2 - so no surprise there.