My first one was 2 years old, believe it or not. A silkie roo, the one I hugged every night.
I first noticed he was falling off the ramp to the coop. Then, he wouldn't try at all. I put him in my hospital cage on the patio. He was eating, but was losing weight. Then I noticed his eye looked cloudy. He had leg paralysis, then his neck would not go where he aimed so he couldn't really eat. And all this time, he crowed every morning. I had the vet do it.
He was the only adult that showed all the signs. The vet misdiagnosed him as having Eastern Equine Encephalitis. So I thought it was a one time thing.
Over the next 2 years, every 3-4 months, one of my adults would waste away. I had no idea why. I thought the others were keeping them away from the food , but that wasn't it. I didn't have a clue till my silkies hatched 10 Polish for me. At 8-16 weeks they all died, most having paralysis.
The one that came to my flock with it was a 3 month old silkie pullet. She was in with the silkies, and only 1 of 4 got it. The rest are still alive, even some of their own chicks that they hatched. I've had hatches in with them, or in a pen near them , and about 1 out of each hatch would waste away at about 5-8 months old. But not the rest. What put it all together was those 10 Polish that were hatched by my silkies, and most were with paralysis. Then I knew.
Now I hatch every one in my incubator, and vaccinate or buy day olds vaccinated, then I quarantine them for 10 weeks. That;s going to be the way for me.