  Planets like the Earth cannot form unless elements heavier than helium are
available. These heavy elements, or `metals', were not produced in the big
bang. They result from fusion inside stars and have been gradually building up
over the lifetime of the Universe. Recent observations indicate that the
presence of giant extrasolar planets at small distances from their host stars,
is strongly correlated with high metallicity of the host stars. The presence of
these close-orbiting giants is incompatible with the existence of earth-like
planets. Thus, there may be a Goldilocks selection effect: with too little
metallicity, earths are unable to form for lack of material, with too much
metallicity giant planets destroy earths. Here I quantify these effects and
obtain the probability, as a function of metallicity, for a stellar system to
harbour an earth-like planet. I combine this probability with current estimates
of the star formation rate and of the gradual build up of metals in the
Universe to obtain an estimate of the age distribution of earth-like planets in
the Universe. The analysis done here indicates that three quarters of the
earth-like planets in the Universe are older than the Earth and that their
average age is 1.8 +/- 0.9 billion years older than the Earth. If life forms
readily on earth-like planets - as suggested by the rapid appearance of life on
Earth - this analysis gives us an age distribution for life on such planets and
a rare clue about how we compare to other life which may inhabit the Universe.
