I haven't seen them yet. except at the open move of the game.

What level are you? I'm at Level 30 and just saw my first one. They appear late in the game...I think someone said Level 28 but I'm not sure. That's an NCR Veteran Ranger.

Added: I guess I wasn't looking hard enough.

Complete your first faction quest, after Ring A Ding Ding. You need to get the warnings about continuing to work with your chosen faction.

I found them at Camp Golf sometime around level 15.

I'm starting to doubt that they appear at a certain level because i've seen them between 10-30 on diffrent playthroughs but they would only appear after i visited the fort.

I've had them appear before the fort.

im seeing them in all ncr camps,at least 3 of em in each ncr camp/base

You have to start the end game quests before they'll start appearing. After you've dealt with Benny you'll need to pick a faction to work with before the Ranger Vets will start popping up.

They appear at levels much lower then 28 it not about your level but how far you gone into the main quest as at level 10 I saw them