“The amount of time that has to elapse before half of the knowledge or facts in a particular area is superseded or shown to be untrue.”
Source: https://en.m.wikipedia.org/wiki/Half-life_of_knowledge
More recent estimates put the half-life of an engineer (software or hardware) at roughly 2 to 5 years. Which means an IT professional would have to spend roughly 10 hours a week studying new knowledge to stay current.
Related to Lifelong Learning, agism in technology (younger people have more recent knowledge gain and simpler lives allowing focus and are hired for less pay).