Actually in IRNs the beak color can be directly related to what subspecies they are. Some subspecies will have a much darker beak color than another. As far as age - I don't know. I have never really paid that much attention. I would think that a good diet could make a difference in the color just as it would in the sheen and shape of their feathers.
As you can see in this picture - they both have different colors on their beaks and both are birds that are over 2 years old. The green one is actually older than the grey one. A lot of young babies will have quite a bit of black on their beaks which fades in time. It just seems to depend on the bird.
