  We review the possibility that metallicity could provide a diagnostic for the
age of a galaxy, hence that the most metal-poor star forming galaxies in the
local universe may be genuinely young. Indeed, observational evidence for
downsizing shows the average age of the stars in a galaxy to decrease with
decreasing mass and metallicity. However, we conclude both from observational
and theoretical viewpoints that metallicity is not an arrow of time.
Consequently the most metal poor galaxies of our local universe are not
necessarely young. Current observations suggest that an old stellar population
is present in all metal-poor galaxies, although a couple of cases, e.g. IZw18,
remain under debate. Further observations with more sentitive equipement should
settle this question in the coming years.
