Neither is 71, 72, 73, 74 and so on, and then one day you really were too old to drive. In a country where driving is a necessity there is no way around this so you get a lot of people driving who really shouldn't be. To play it safe I've set the limit at 70 for myself because I think that if I don't put a hard line down I will always think 'one more year'. This avoids the gray area entirely, exactly because 70 isn't too old to drive.
I know a 62 year old that shouldn't drive. And I know a 76 year old that is fine. It really depends upon the person, but our dmv isn't fit to detect this.