My grandfather was born in the 30s and only died like a year or two ago, and I'm on the younger end of his line (my oldest cousin is about to be a grandma, and I'm a millennial).
To fight in WW2 you would have to be born in the early 1920 (to fight at the start of the war) or the mid 1920s at the latest, so I would say about 100-105 years old or roughly 2 1/2 generations of very late parents (having kids around 40) ago.