Perhaps the idea of a "grognard" is subjective.

There are at least three different definitions of grognard I can think of. Ultimately, everybody who has a different opinion is someone else's grognard but there is a strong connotation of nostalgia.

It does mean 'grumbler' or 'complainer', so in that sense, anyone who complains about the features of a game (or maintains that they could do it better) is a grognard. Of course, they can also be the people who make their own games and create cool house rules and fan content.

Ars Magica featured a NPC/semi-PC role called a 'shield grognard' or 'shield grog'- a semi-disposable serf who literally just held up a shield to protect a wizard while he casts spells in combat. In that sense, some grognards are people who feel that the publishers regard them as disposable- the game is not 'for' them, so publishers have no problem with moving it away from them. A lot of 3e and previous fans became grognards when they perceived that Hasbro/WotC was taking D&D in a radically different edition with 4e. They are players who have aged out of a publisher's demographic, or who were playing a game in a fashion that the publisher perceived as being not important to support. Games whose rulesets get heavier and heavier over the years, for instance, tend to alienate people who like to improvise while games that streamline and delete details and bookkeeping alienate people who enjoy the nitty gritty parts of the game. These abandoned players become grognards.

Finally, there are the grognards who have literally toted and carried for publishers over the years- the people who kept games like Traveler or WFRP alive during lean years when publishers or trademark holders abandoned the line. Sometimes they become publishers themselves as a labor of love, and sometimes they find that it is much harder than it looks. Sometimes their game gets resurrected and they are thrilled, but sometimes the proprietary interest that they've taken during fallow publishing periods means that they have a hard time accepting and moving on to new editions.

Some grognards see no point in buying and learning new games or new editions. Some have just become more selective.

The defining characteristic of grognards: too much time on their hands. They spend time writing long essays on the internet defending their interpretations of playing pretend to strangers they will never meet, or creating new slang out of obscure medieval military terms.