yes I think it is an old age I have a very old female guppy and she used to be bright yellow kind of glowing on the light I have been watching her last night and she is turning white, at first I thought she has white stains but it really does look like she is just loosing color and looks not that shiny as before... all my other guppies look great except 2 other females that are her age they look like they are getting skinnier and i can tell one of them has her spine getting curved, i was told in this forum this also happens when they werent bred well and it shows up when they get old ... poor poor old guppies...