0-LEVEL - Novices (non-grognards): Started playing during the 5E era, 2014 or later. Generally under age 25 - younger Zennials and Alpha.
BASIC - Apprentice grognards (aka "quasi-grognards"): Started playing during 4E, age 25-35ish, or older Zennials.
EXPERT - Journeyman grognards: Started playing during 3.x, probably age 35-45ish. Younger millenials.
COMPANION - Adept grognards: Started playing AD&D 2nd or BECMI, 1990s. Age 40-50ish. Millenials, mostly.
MASTER - Master grognards: Started playing AD&D 1st edition, or B/X D&D. Age 50-60ish. Gen-X or older.
IMMORTAL - 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.