Delays diagnosing and treating children with arthritis are leaving them in pain and at a higher risk of lifelong damage, a national charity has warned.
Arthritis is commonly thought to affect only older people, but 15,000 children have the condition in the UK.
Versus Arthritis says many children are not getting help soon enough.
The NHS said: "Arthritis in young people is rare and diagnosing it can be difficult because symptoms are often vague and no specific test exists."
Eleven-year-old William Riddler, from Ripon in North Yorkshire, has had symptoms since he was three, but was not diagnosed with Juvenile Idiopathic Arthritis (JIA) until he was nine years old.
He now has it in most of his joints.
"I wish people could see the pain, like the pain in my arms, because people don't see it, so sometimes they don't believe you and then they just start laughing or something," he said.
"I wish people could see it to understand because people normally think just older people can get it and younger people can't."
Zoe Chivers, head of services at Versus Arthritis, said: "We know that young people often face significant delays getting to diagnosis simply because even their GPs don't recognise that it's a condition that can affect people as young as two.
"It's often considered that they're just going through growing pains or they've just got a bit of a viral infection and that's not the case."
While the exact number of children affected by a delayed arthritis diagnosis is not known, Versus Arthritis said many of the families it supports have had similar experiences.
William's mother Louise Riddler said: "They [doctors] put it down to just a virus when his knee swelled up.
William went on to have numerous X-rays and tests but it was only after an ultrasound was carried out on his thumbs, that he was diagnosed with arthritis.
"I think if William had been diagnosed at three he would have been able to have a simple steroid injection. Not being diagnosed till nine, by that time, he had arthritis in quite a lot of his joints."
Dr Rachel Tattersall, a consultant in young people's rheumatology in Sheffield, said: "The sooner we recognise and treat arthritis, the easier it is to reduce the disease process with standard medicines. More established disease may have already caused damage and is more difficult to treat."
An NHS spokesperson said: "NHS services across the country are committed to providing timely care to any child who needs it, and any individual concerns about delays should be raised with the relevant team."
What is arthritis?
Arthritis is a common condition that causes pain and inflammation in a joint. In the UK, more than 10 million people have arthritis or other, similar conditions that affect the joints. Arthritis affects people of all ages, including children.
Most types of childhood arthritis are known as Juvenile Idiopathic Arthritis (JIA), the symptoms often improve as a child gets older, meaning they can lead a normal life.
There's no cure for arthritis, but there are many treatments including medication, physiotherapy and surgery that can help slow it down.