The statement appears on page 965 of William Lyon Phelps's _Autobiography
With
Letters_. The statement does not appear in quotation marks, nor does Phelps
cite it from a letter. Phelps writes:

Mark Twain said life would be infinitely happier if we could only be born at
the age of eighty, and gradually approach eighteen. This would mean we
should
gradually lose our intelligence, our experience, our ideas, our work, our
personality, even our manhood. We should exchange profound happiness for
animal spirits (p. 965).

Phelps knew Mark Twain and appears to be recalling the statement from
memory.

Barb