What would you do if you were diagnosed with arthritis in your 20s?

But it's a growing issue for millennials, according to new research from Monash University and Arthritis Australia, which predicted one in six millennials will be suffering from arthritis by 2040.

Mr McMahon said he first started noticing symptoms in his knees in his early 20s.

"Eventually I got an official diagnosis in 2017 when I was 29 ... there was just constant pain and swelling regardless of what I did, it wouldn't go away," Mr McMahon said.

"I had trouble walking upstairs and downstairs, even sitting or standing - just really simple things that shouldn't bother you."

While he has a family history of the disease, the diagnosis still came as a surprise.

"My uncles and my dad found out when they were in their 50s, so it definitely wasn't something I expected when I was in my early 20s," Mr McMahon said.

"I always associated it with something that older people would get and I was fairly physically active at the time as well."

Mr McMahon was pursuing a career as a paramedic when he got the diagnosis, and was told by a doctor to abandon the career as his knees weren't fit for the job.

He now works as a research nurse to accommodate for the lifestyle, but said the changes hadn't come without sacrifices.

"I also eventually gave up being a clinical nurse because being on my feet all day was fairly hard on my knees as well," he said.

"And effectively just gave up all sorts of team sport that I'd been playing for years and years.

"It was hard, I effectively had to give up everything all at once just to get the pain and swelling under control."

In terms of treatment, Mr McMahon has had an arthrascope on both knees to clean out loose cartilage, and regularly attends physio, pilates sessions and strength-based training.

"I was told to avoid the exercise bike and that I shouldn't be doing squats or lunges, but since the last two to three years of training, I can squat, deadlift and stand for longer periods of time," Mr McMahon said.

"I still can't run, that puts too much pressure through the joint, but I can do a light jog to run after my son if I need to."

Monash University professor llana Ackerman said despite being a common disease, rheumatoid arthritis was particularly misunderstood.

"Many people are unaware that it is an autoimmune disease that causes pain, disability, severe fatigue and brain fog, while also potentially damaging eyes and other organs," Dr Ackerman said.

"Australians with osteoarthritis currently face major gaps in access to care according to clinical guidelines, which if properly implemented could improve health outcomes while reducing the need for expensive joint replacements."