Some times the time table to a forager gets moved up. This happens if there is a drop in the number of foragers. When this happens the life span of the bee is shortened.
They move up as the need arises
honeyshack and 11x are correct - this is a biological process termed, "recruitment" (and it is not restricted to bees - for example, some colonial nesting birds, such as beeeaters, recruit recently fledged siblings to assist in raising the new clutch - instead of reproducing themselves).
My understanding is that, among bees, it is a result of some combination of age and the physiological conditions within the hive.
Randy Oliver wrote an interesting piece a couple of months ago on how bees are recruited to bringing in more water when the hive requires. Recruitment and maintaining individuals as water foragers is, in part, dependent upon unloading times at the hive entrance.
Apparently, if hive demands for a particular task exceed the available work force (in age class), other nearby age classes are recruited to that task. Once the 'need' is satisfied, the bees resume their 'age specific' tasks once again.
As they progress in age though, it is thought that they reach points physiologically whereby it is more difficult for them to perform certain tasks any longer, such as foragers reverting to some house bee tasks - although, as 11x says - it does happen.
This is a fairly active area of bee research though - so who knows what might be learned about it in the future.