Although we are never given an exact age, we know that Momaday's grandmother lived to be very old. The reader knows this by the following quotation:
Now that I can have her only in memory, I see my grandmother in the several postures that were peculiar to her: standing at the wood stove on a winter morning . . . sitting at the south window, bent above her beadwork . . . going out upon a cane, very slowly as she did when the weight of age came upon her; praying. I remember her most often at prayer.
There are many important words here that lend themselves to the idea of old age. Momaday's grandmother, Aho, is "bent" over her beadwork. This shows that her spine is no longer straight (which often accompanies old age). Next, she always goes "out upon a cane, very slowly." Using a cane and moving "very slowly" are also usual aspects of an elderly person. Finally, Momaday gives the direct description that the "weight of age came upon her." This is a literal way of telling readers that Aho is truly old.
Further, Aho's age is extremely important in that it is her age that allows her to have experienced some of the most important events in Kiowa culture. Most importantly, Aho experienced the last two Sun Dances of the Kiowa tribe of Native Americans. Aho describes both for her grandson, Momaday, who eventually writes this book. In a sense, it is The Way to Rainy Mountain that immortalizes the Kiowa tribe in regards to myth, history, and personal experience.