Hal,

Here is some additional information from Thomas A. Tenney's "Mark Twain: A
Reference Guide, Sixth Annual Supplement" in _American Literary Realism,
1870-1910_,
Vol. XV, Number One, Spring 1982, page 20:

1979

Crosland, Andrew. “Sources for Fitzgerald’s ‘The Curious Case of Benjamin
Button,” _Fitzgerald-Hemingway Annual_, pp. 135-139.

Fitzgerald said the idea of a reversed life “was inspired by a remark” of
MT; apparently the source was a passage in the Paine biography (1912.A3,
vol. 3, 1440-1441). Fitzgerald would have known of this passage from reading
in Brooks’s _The Ordeal of Mark Twain_ (1920.A1, p. 175), a copy of which he
had been given by Maxwell Perkins: “It was one of Mark Twain’s favorite
fancies, Mr. Paine says, that life should begin with old age and progress
backwards.”

~~~~~

And from the Paine biography, Vol. III, page 1440-1441.

“He regarded the decrepitude of old age as an unnecessary part of life.
Often he said:

     ‘If I had been helping the Almighty when He created man, I would have
had Him begin at the other end, and start human beings with old age.  How
much better it would have been to start old and have all the bitterness and
blindness of age in the beginning!  One would not mind then if he were
looking forward to a joyful youth.  Think of the joyous prospect of growing
young instead of old!  Think of looking forward to eighteen instead of
eighty!  Yes, the Almighty made a poor job of it.  I wish He had invited my
assistance.’"

~~~~
Cheers,

Kevin B.