This Two Gray Hills was made in 1880. This rug dates back to the early 1800 s. This continued until they returned from Fort Sumner. Nothing that is an out and out rug is older than that. Evaluate the rug using many of the same criteria that you would for a new rug, but remember that the conditions that many older rugs were woven under were truly difficult and standards today are very high.
Older rugs are more likely to be made with handspun, and the designs are more personal.
Uneven carding, faded dyes, and idiosyncratic designs are more common in older rugs. If you are paying a high price for something you're not sure of, get a second opinion. You may have doubts that the rug is Navajo at all. Mexican and Rio Grande rugs are woven on a floor loom, and have a fringe. In order to support the Navajo made claim, floor loom weavers sometimes work the fringe back into the rug, but a careful inspection of the ends will reveal the doubled warps.