How do you tell a guy who has championed cars like the Dodge Viper and Plymouth Prowler that he's too old for the business? If you're Chrysler's board of directors, you don't. Robert A. Lutz, Chrysler Corp. president, is having so much fun, the board is extending his contract two years beyond the company's mandatory retirement age of 65, which he will reach in February 1997. Of course we all remember what happened to the last executive Chrysler allowed to work beyond the "mandatory" retirement age. Former Chrysler Chairman Lee A. Iacocca stayed a couple years beyond his 65th birthday, then wanted to stay some more. The board said "no thanks," lured Robert J. Eaton from General Motors Europe, and Mr. Iacocca has been seeking revenge ever since. But Mr. Lutz, who became Chrysler president in 1991, is Detroit's consummate "car guy," and keeping him on should reassure Wall Street that Chrysler's pipeline will be stocked with solid new products into the 21st Century. Now if Chrysler can just keep him f rom performing those death-defying stunts in his helicopter.