Nolan Ryan, Jamie Moyer, Phil Niekro, Warren Spahn, etc etc are the exceptions not the rule.
However, longevity is a rule with knuckleballers who had success at the MLB level.
Joe Niekro might be more like RA Dickey than any of the others -- to the best of my knowledge. If I recall correctly, Joe threw his fastball regularly. Joe threw about 3800 professional innings until the age of 43, and his career started going south at the 3300 IP bench mark. R.A. is at 2300 professional innings presently. Each situation is unique, but this is one reason why I keep saying R.A. has 1000+ innings left in his arm. What else is there to go on in order to speculate?