  We use numerical simulations of hydrogen reionization by stellar sources in
the context of LCDM cosmogonies to investigate the 21 (1+z) cm radio signal
expected from the diffuse intergalactic medium (IGM) prior to the epoch of
reionization breakthrough at redshift z_ion. Two reionization scenarios are
analyzed in details: an `early reionization' case with z_ion~13, consistent
with the recent discovery by the WMAP satellite of a large optical depth to
Thomson scattering, and a `late reionization' case with z_ion~8. It is a
generic prediction of these models that the background of Lyalpha photons
produced by the early generation of stars which ultimately ionize the universe
will be sufficiently intense to make intergalactic neutral hydrogen visible
against the cosmic microwave background during the `grey age', i.e. z_ion<z<20.
Depending on the redshift of reionization breakthrough, broad-beam observations
at frequencies <150 MHz (below 100 MHz for the `early reionization' scenario)
with the next generation of radio telescopes should reveal angular fluctuations
in the sky brightness temperature in the range 5-20 mK (1 sigma) on scales
below 5 arcmin.
