Part 1: What is a Grognard?
First, a new definition of grognard: It starts with anyone older than the target audience age of D&D. Now of course that is rather nebulous, but I would think the target audience is the big part of the bell-curve of current D&D players, which is probably something like age 15-25 right now (could be wrong, but that's my guess...I'm guessing someone comes in here and says, "I disagree, it is actually probably more like 14-24"). Furthermore, under my definition, a grognard must have cut their teeth on an older edition of D&D, meaning 4E or older (2014 doesn't count).
Now I would put for that grognardism is a spectrum. We could posit the above definition equating with "quasi-grognards" or perhaps "apprentice tier" grognards, while journeyman grognards are two editions removed, and so forth. Something like this:
Novices (non-grognards): Started playing during the 5E era, 2014 or later. Generally under age 25 - younger Zennials and Alpha.
Apprentice grognards (aka "quasi-grognards"): Started playing during 4E, age 25-35ish, or older Zennials.
Journeyman grognards: Started playing during 3.x, probably age 35-45ish. Younger millenials.
Expert grognards: Started playing AD&D 2nd or BECMI, 1990s. Age 40-50ish. Millenials, mostly.
Master grognards: Started playing AD&D 1st edition, or B/X D&D. Age 50-60ish. Gen-X or older.
Grandmaster grognards: Playing with Gygax in the 70s, OD&D, baby. Age 60+. Boomers or older. Of course a true grognard had to have played wargames before D&D even started, but we'll waive this criteria for grandmastery.