I have osteoarthritis both knees, first spotted by xrays & MRI when I was 11. Arthroscopy left knee 6 years ago (age 42) removed most of the cartilage. I do not recommend surgery if you can avoid it - its only value was that it made me take the post-op physio seriously! And it was physio that "saved" me and kept me cycling. (A custom bike that actually fit me, didn't hurt either!)
Addressing muscle weakness/tightness/imbalances may be the single most helpful thing you can do for your knees (takes pressure off the joint, ensures good tracking, etc). Prolonged standing/walking/stairs will probably always be a problem for you but exercise does help.
Pilates can help too if it's the right mix. It's hard to get "circuit" Pilates classes outside London, so beware those offering exclusively matwork or Reformer work, as they may overwork muscles that are already quite strong from cycling while not engaging the counter-acting muscles that often don't get a look-in from cycling but are essential to get engaged and working properly if your joints are to be well supported.
No idea what part of the country you are in but if you can afford to go private, I highly recommend the physios at Balance in Clapham and also Nicole Oh who practices in Putney and Blackfriars (as recommended by
@vickster in other threads - thank you
@vickster!!)
Oh and that "you're too young for knee replacement" is total bollocks in this day and age. The materials used now will outlast you. My mother suffered for decades being told she was too young and "come back in 10 years". At age 60, the orthopedic surgeon said "OMG, why have you not come in sooner?" and she burst into tears and wailed "because you all said I was too young and not to bother you!" My sister had both knees replaced at age 52. The orthopedic surgeon and Xray technician said her knees looked like they belonged to someone in her 80s. She followed physio to the letter and went off on a walking holiday 6 weeks post-op and was back to high alpine hiking/climbing just a few months later.
Also don't believe any surgeon who says "if you have knee replacement, you'll never cycle again". (My surgeon told me this, scared the sh*t out of me -- see above comment about the arthroscopy making me take physio seriously!!) Quick Googling will turn up cyclists who had bilateral TKP and were doing 200km audaxes within 6 months.
Good luck!