My theory on Duncan's longevity: he's relied on his athleticism probably less than any other star bigman in history, which has put limited the mileage on his knees. Yea he's had his share of dunks and coast-to-coast plays over the years, but he was never like Lebron or Griffin or whoever. From day 1, he approached the game almost like it's a brain teaser. Set up in the post and then use footwork and fakes to confuse his man, get better position, or get space for a shot. Obviously strength and athleticism play a part in that, but Duncan used sound fundamentals and brain power more than anyone.
Remember that for most guys the late 20s to mid 30s are our physical prime. Professional athletes in basketball & football are different because of the wear and tear on joints the sports put on the joints, not because they're too weak or too slow. Examples: the Kona Ironman all time record was set in 2011 by a 37 year old. Tons of guys that compete in the strongest man in the world contests are in their mid 30s (I know they're roided out to hell, but there are plenty of roided out dudes in their mid 20s). Carl Lewis won a gold in the 1996 Olympics at age 35. And that dude in the Bowflex commercial is 43 and in the best shape of his life.
Cliff Notes: it's the wear and tear on knees that causes basketball players to decline, not the fact that 35+ is too old. Duncan's style of play has limited said wear and tear, meaning he's got more mobility than most guys his age.
[This message has been edited by Deputy Travis Junior (edited 5/28/2013 4:40p).]