We often use "generation" in this way. For example, we might say, ‘The soldiers of Napoleon’s generation knew nothing about airplanes and atom bombs.’ Would we be referring just to soldiers who were born in the very same year as Napoleon was? Would we be referring merely to those French soldiers who died before Napoleon did? Of course not; nor would we by such a use of "generation" be trying to fix a set number of years.
...It is similar with our understanding of what Jesus said in his prophecy given on the Mount of Olives.
The way the Watchtower wants to use the "generation of 1914" is to say that we are still the people of Napolean's generation. At some point "generation" will break, it can't encompass everyone who lived since 1914. The First World War generation includes people around the period that started during the Victorian age (another generation long gone). It does not inlcude the post WW II, baby-boomer generation. My grandfather, who died before I was born, was born around 1890 and served as a soldier before and during WW I. He could be said to be of the 1914 generation. My father was too young to serve in WW II and I am a baby-boomer. I don't count myself as part of the 1914 generation anymore than I am part of Napolean's generation. 100 years after 1914, "this generation" will be a bit passed its sell-by date. Expect more new light by 2014.