So, as you might have gathered, the oldest GPS satellites currently up there and operating are upwards of 20 years old, which is certainly a long operating time for a satellite. They're getting old. They've functioned beautifully during their lifetime, but they're getting old. No matter how well something is built, long term wear and tear in the harsh environment of our upper atmosphere (GPS satellites are in a low orbit) will strain any system, and they're starting to feel it. One report put it, in oft-quoted words, that they are “close to breakdown," putting the beginning of failure in 2010. Uh-oh.
Repairing them would be near impossible, and the GPS system has proven simply too valuable to allow to fail, leaving replacement as the only option. The US Air Force, the entity in charge here, had put plans into place to have new satellites up and running in 2007, but that date's long gone, leading to cries of neglect and bad management on their part—including by the Government Accountability Office (GAO).
It should also be kept in mind that in addition to the 24 GPS satellites currently in service, there are an additional 3 in orbit in case any of the 24 break down. Also, the Block II satellites are far from being of a uniform age: the satellites of that group that are in imminent danger of breaking down are the older ones, while the newer ones, less than half their age, have a few years left in them. A total breakdown of the system is quite unlikely in the immediate future.