  The wide luminosity dispersion seen for stars at a given effective
temperature in the H-R diagrams of young clusters and star forming regions is
often interpreted as due to significant (~10 Myr) spreads in stellar
contraction age. In the scenario where most stars are born with circumstellar
discs, and that disc signatures decay monotonically (on average) over
timescales of only a few Myr, then any such age spread should lead to clear
differences in the age distributions of stars with and without discs. We have
investigated large samples of stars in the Orion Nebula Cluster (ONC) using
three methods to diagnose disc presence from infrared measurements. We find no
significant difference in the mean ages or age distributions of stars with and
without discs, consistent with expectations for a coeval population. Using a
simple quantitative model we show that any real age spread must be smaller than
the median disc lifetime. For a log-normal age distribution, there is an upper
limit of <0.14 dex (at 99% confidence) to any real age dispersion, compared to
the ~=0.4 dex implied by the H-R diagram. If the mean age of the ONC is 2.5
Myr, this would mean at least 95% of its low-mass stellar population has ages
between 1.3--4.8 Myr. We suggest that the observed luminosity dispersion is
caused by a combination of observational uncertainties and physical mechanisms
that disorder the conventional relationship between luminosity and age for pre
main-sequence stars. This means that individual stellar ages from the H-R
diagram are unreliable and cannot be used to directly infer a star formation
history. Irrespective of what causes the wide luminosity dispersion, the
finding that any real age dispersion is less than the median disc lifetime
argues strongly against star formation scenarios for the ONC lasting longer
than a few Myr.
