All ball pythons will darken with age ... normals, pastels, albinos, axanthics, etc ... in scientific terms it's called an ontogenic color cage ... many other species go through the same thing ... I don't think I've ever seen anything that talks specifically about the "whys" and "hows", but if I had to speculate, I would guess that it is caused by a combination of diet and hormones over the lifetime of the animal.

Some pastels "change" more than others ... a very very nice looking hatchling pastel has a much better chance of still looking very very nice as an adult ... a "but hurkting" hatchling will not get "better" as it matures and will most likely still look "but hurkting" as an adult. Also, some lines of pastels generally hold on to their colors better into adult hood than others.

-adam