Good question!
I really don't have an answer for you since I only have 1 amel stripe, and he's only 2 yrs old now. I really haven't noticed much of a change in his coloration/definition of his stripes over the past year, but he is still fairly young. I do know that alot of motleys (especially normals) lose some of their pattern definition when they reach maturity, but not all. And then, some stripes do have a fading effect to their pattern. And many corns generally get darker, especially their background color, as they get older anyway, which may be what you're seeing.