Yeah, I want to say they touched on this in the Republic/Imperial Commando series? Basically the master set of DNA from Jango Fett deteriorated over time (and there may or may not have been some sabotage involved) so as the clones started hitting the end of their hyper-shortened lifespans, the Empire needed to start conscripting actual soldiers, as well as cloning from other, non-Jango sources. Basically the average lifespan of a clone was something like 25-35 years, but the rate of development exponentially grew towards the end of the clone's life. So basically old age took a clone in a year or two, instead of a few decades. This was something they were trying to circumvent when Imperial Commando (the book series) got axed. It's heavily implied (if not outright stated) that they had some degree of success, as Jaing Skirata, an ARC trooper, survives to 40 ABY and shows up in the Legacy of the Force series.