alt Hacker NewsLost 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.
In the US, something like 98% of eligible men enlisted in WWI. There was no draft.
So yes, VERY high probability of service.