If you were born in 1900 you probably are at the tail end of the Lost Generation — the Greatest Generation is considered to be those born between 1901 and 1927.

https://en.wikipedia.org/wiki/Greatest_Generation

Wild to think there were people who as adults lived through all of the railroad buildout, WWI, the 20s, the depression, and then WWII. Complaining about AI buildouts causing electronics prices to regress by a decade or so begins to seem rather trite in comparison.

Lost Generation describes those who experienced WW1. Given that he turned 18 in 1918 it's certainly possible he enlisted or was drafted. The article implies he didn't join WW1. It's that experience rather than his exact birthday that would categorize him into Lost vs Greatest IMO.