@ Eldoran; That's kind of a strawman, but hey I'll work with it. Let's discuss it further if my point is at all unclear.

Yes, Thrall had a massive role in how orc history unfolded, and his effect on their culture has been prolific. But not every orc is going to have the same experiences and the same opinions about their racial history, and you can't say that an orc of age group X is going to be "fiercely loyal" to Thrall anymore than you can say that a 40 year old woman is "likely" to vote Republican... because there is much more to consider than age. People are more complicated than that, and orc history and culture is more complicated than that.

Let's consider how an orc who has lived through all the lows of history might dislike Thrall. Heck, let's take a canon character and consider what his life might have been like, and thereby demonstrate that an orc roleplayer playing an orc of 40+ who is not fiercely loyal to Thrall isn't breaking some kind of unwritten rule.

Overlord Agmar was a pretty important npc in Northrend. We can assume, by his white hair, that he is old enough to have lived through all the important bits of orc history. We also know that, despite not being a wet-behind-the-ears youngblood, Agmar is 100% on board the Garrosh train. How can we justify this?

What if Agmar was from the Warsong Clan? As an old Warsong orc, he would have been raised in one of the most militant groups of orcs, and spent a great deal of his life following the audacious, headstrong and reckless leadership of Grom Hellscream. As a Warsong orc, he would come from an environment wherein Grom Hellscream was considered an ideal type of orcish leader. As a Warsong orc, he would have sat out the First War and most of the Second War, instead living in hardship on a dying Draenor that was increasingly desolate and hostile to life. As a Warsong orc, he not have been placed in internment alongside the other clans, but would instead have eked out a life of hardship in the wilds of Lordaeron, spending even more years living in hardship in the wilderness. He would have lived through those years, when Warsong infants rarely survived the Lordaeron winter, enduring the slow, creeping death of his clan against all the odds, probably surviving at times on nothing more than rats and boiled leather. And, as a Warsong orc, he would have endured those years without succumbing to the same lethargy that plagued the orcs in internment... largely due to Hellscream's passion and strength of will.

Perhaps, at the conclusion of the Third War, this Warsong orc would have thought "Finally, we have a new home, and we can build a new future. Our times of hardship are done, and I will not have to watch one more orcish infant die from hunger in a Barren wilderness". And then... Thrall settles his people in Durotar, preaching that the orcs have to atone through hardship and suffering. The Warsong orc has flashbacks of those years in Hellfire peninsula, of the wilds of Lordaeron, of hunger and hardship and a fearful, uncertain future for his people... and he sees it all coming back. Everything his people could need is conveniently nearby in the bounty of Ashenvale, but instead of settling there our Warsong orc sees that Thrall keeps them confined to a wasteland and puts the best interest of the orcs second to the capricious favour of foreign powers.

And then, this Warsong orc hears that the son of Hellscream is alive, and sees him in the flesh when he returns. Garrosh preaches non-compromise, has grand ideas of the destiny of the orcs to thrive rather than to endure in hardship, and he leads like Grom would lead. This elderly Warsong orc, because of all that he has seen, endured and lived through, and because of his clan culture and wartime experience, is likely to feel great loyalty to Garrosh and at uninspired loyalty towards Thrall. And if a roleplayer played his orc like this, he wouldn't be wrong for breaking some unwritten rule.

With a bit of knowledge about orc history and culture, you could concoct a similar backstory for a Blackrock orc, a Shattered Hand orc, even for a Frostwolf orc, and develop your character in a way that explores the political and ideological opposition to Thrall. Because orc history and culture has so much substance, you need to consider all sorts of things like clan affiliation and wartime experience before you can presume to know the mind of an individual orc. "Mature orcs = Thrall, Young orcs = Garrosh" is a massive oversimplification of a diverse and complicated racial group, and it you just can't apply to the individual roleplaying character as a presumptuous rule.