To me, age is just a number. What is really at stake is your health; and then there are those who at any age, aren't in the best of health, but throw caution to the wind and say, "I'm not going to live forever, so I'm going to see the world!" I have met many elderly travellers, and you might be surprised at the number travelling RTW, however you won't meet too many in the low-budget backpacker guesthouses.
My favorite example is Jeanette, a Dutch woman who was 67 when I met her in the western deserts of China. She had backpacked on most of her trips, and she was carrying over 20kg (44 lbs) on her back this time. The next time I spoke with her, she had been on a camel safari with Bedouins in the Sinai, and skiing in the Alps.
Road Scholar / Elder Hostel
Walking the World
Resources for Disabled Travelers at Rick Steves' Europe
Lonely Planet Forum for Older Travellers