I had always been told that the orange showed up as they got older, but after your response I did some research on the net. All the sites I looked at said that the orange shows up during breeding season on mature males but it didn't give any age for when they reach that maturity. It also said that if your iguana keeps the orange coloring year round - they are most likely dehydrated or could have kidney problems.
Spike, my male, never had any orange on him until just a few years ago - but now he has alot of orange and it never goes away. I'll be doing the other tests to see if he's dehydrated.
Here is the most comprehensive site that I found: