"Jesus was obviously speaking about those who were old enough to witness with understanding what took place when the 'last days' began. ... Even if we presume that youngsters 15 years of age would be perceptive enough to realize the import of what happened in 1914, it would still make the youngest of 'this generation' nearly 70 years old today." Awake! 1968 Oct 8 p.13
Growing up I used to view WWI vets as a definitive marker. Many of these men went off to war not much older than 15. I had no idea they'd all died off. At one point I was certian there had to be some around when armageddon came.
On the other hand, it'll probably take another 15 to 20 years before everyone born on or before 1914 is dead.