Just out of curiosity, Art, where did that figure come from? I've
looked for surveys which specifically asked about the age of the
Earth, and never found one.
I am aware of a few surveys of high-school students, from which
some numbers may be derived. In Eve and Harrold (1991), for example,
it is reported that about 1/4 of high-school students agreed with
the statement "God created humanity pretty much in its present form
in the last 10,000 years or so."
Since it is possible to agree with that statement and still accept
an old Earth (a la R.H. Brown), but hard to disagree with it and
accept a young Earth, I would treat 25% as an upper limit for the
young-Earth fraction of the sampled populations.
Eve, Raymond A., and Francis B. Harrold, 1991. _The Creationist
Movement in Modern America_. Massachusetts: Twayne Publishers,
ISBN 0-8057-9742-4 (paper). The surveys on recent creation
of humans are reported on p. 33.