John Roberts, if confirmed as Chief Justice, will be the youngest Chief Justice in over 200 years. Beginning with John Jay in 1789, there have been 16 Chief Justices of the Supreme Court.
The average age of Chief Justices since 1802, at the time, of swearing in, is 61 years, 154 days. Roberts is 50 years, 8 months: over ten years younger than this average. The youngest Chief Justice in the last 200 years was Melville Fuller, appointed by Grover Cleveland at age 55 years and nearly 8 months (five years older). In terms of life expectancy, Roberts would be by far the youngest Chief Justice of the Supreme Court.
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The sixteen Chief Justices have served an average of 13 years, 3 months. In recent history, there have been three Chief Justices since 1953. By life expectancy, Roberts should live over 30 more years.
I posted a more extensive analysis of the age of Supreme Court justices here:
Stacking the Supreme Court
http://www.dailykos.com/story/2005/8/15/11115/9557