I have been looking into this and honestly, figuring it out has been surprisingly difficult. There are far fewer adult morph pictures running around than I would have believed, far more baby/juvenile pictures.
It's hard to tell these pythons age just from the photos, but it does appear that some "larger" ball pythons keep their coloring after they age some. That said, it does seem like almost all lighten up some.
http://ssscales.com/wp-content/uploa...3570498_n1.jpg