What Aragorn says...

[Willis]

So in the Rivendell book in the ranger section, it says "what Aragorn says" in the section where the leader of the culture discusses the other cultures. I thought Aragorn didn't learn about his heritage until 2951, which is after the point the game takes place in the timeline. Am I mistaken or is there something else at work?

[Fenton Hardy]

The authors used Aragorn as a convenient stand in for the Rangers generally. I am not sure who else they could have used. Arathorn II was dead before the events dealt with by TOR. Halbarad, I suppose, but he less well known and I don't know if he is older than Aragorn. I think it had to be Aragorn.

[Willis]

Understandable. I was just making sure I hadn't missed something in my lore somewhere. At the start of the campaign, though, he is still Estel in Rivendell, correct?

[Rich H]

Am I mistaken or is there something else at work?

You're mistaken. Page 5 of Rivendell states the following:

The Passing of Years
The material presented in Rivendell assumes the year is some time after 2951, possibly as far into the Tale of Years
as the year 2977, the ideal date for the conclusion of The Darkening of Mirkwood campaign.

That's why Aragorn is used.

[Willis]

So would I be unwise to use the cultures before 2951.

[Rich H]

So would I be unwise to use the cultures before 2951.

Depends on your game, only you can answer that, but Rangers and Elves (of Rivendell) exist before this time so there's no reason why they can't be used based on any in-game reason as they are active in the region*. Using them is more about your own requirements for balance with other cultures and the like and what you're trying to do with your campaign. If you and your players aren't experienced with playing TOR I wouldn't recommend using either culture, though.

* The page I quoted continues with this:

For Loremasters who wish to set their games in Eriador to be contemporary with the start of The One Ring campaign (TA 2946), note that life has changed little in these lands in the past few decades.

[Otaku-sempai]

One Caveat: I doubt that Aragorn II fully took his place as the leader of the Dunedain of the North until after TA 2980. From 2951 up to that point he was traveling in disguise to various regions of Middle-earth, learning about their peoples and customs, and serving Rohan then Gondor under the name of Thorongil.

[Fenton Hardy]

I was reading Concerning Archers from Ruins of the North, the companion to Rivendell, just before the OP and that adventure can begin as early as 2946, which is why I gave my original answer.

[Otaku-sempai]

Well, someone had to be the acting leader of the Dunedain in Eriador during the years between Arathorn's death and TA 2980. We just don't know who was Arathorn's second-in-command or who might have succeeded him before Aragorn came of age. Halbarad of the Grey Company is just my (and Fenton's) best guess.

[Stormcrow]

Halbarad of the Grey Company is just my (and Fenton's) best guess.

Only because we don't have any other names to choose from. You might just as easily make up a new character for that spot.