Terry Tao, mathematician.
Aged 2: could do arithmetic.
Aged 8: SAT score of 760 in math.
Aged 9: Attending university courses in mathematics.
Early teens: youngest ever medal winner in International Mathematical Olympiad (punishingly hard competition).
Aged 16: got both Bachelors and masters degrees.
Aged 20: PhD.
Aged 24: Youngest ever full professor at UCLA.
Aged 31: Winner of Fields Medal (sometimes called the "Nobel Prize of Mathematics").
Now (aged 37) happily married with a child, possibly the greatest living mathematician, and claims that some of his best ideas come while he's chatting to other parents waiting to pick up his child from day-care.
To me, Tao embodies genius. He has a mind so astonishing that he seems several levels more evolved than me. And he is a child prodigy who has seamlessly moved into adulthood while maintaining his genius and his humanity.
http://en.wikipedia.org/wiki/Terence_Tao
Whether he's an exception I don't know, but he does show that not every child prodigy burns out by adulthood.
If you want peek at his mind, read his blog:
http://terrytao.wordpress.com/
-T.