Ref: http://physicsweb.org/article/news/7/7/4

Astronomers believe that the Moon was formed when a Mars-sized body
smashed into the Earth, ejecting matter into orbit and lengthening our
day to its present value of 24 hours. Until recently, however, estimates
of much of the Moon is "impactor material" that came from this impactor
object, as opposed to the Earth, have varied wildly - from 1 to 90%. Now,
by comparing the compositions of lunar and terrestrial rock samples,
astronomers in Germany have calculated that no more than two-thirds of
the Moon is impactor material. Moreover, they estimate that the Moon must
be at least 4.5 billion years old (C Münker et al 2003 Science 301 84).

See: http://physicsweb.org/article/news/7/7/4