  We fit the colour-magnitude diagrams of stars between the zero-age
main-sequence and terminal-age main sequence in young clusters and
associations. The ages we derive are a factor 1.5 to 2 longer than the commonly
used ages for these regions, which are derived from the positions of
pre-main-sequence stars in colour-magnitude diagrams. From an examination of
the uncertainties in the main-sequence and pre-main-sequence models, we
conclude that the longer age scale is probably the correct one, which implies
we must revise upwards the commonly used ages for young clusters and
associations. Such a revision would explain the discrepancy between the
observational lifetimes of proto-planetary discs and theoretical calculations
of the time to form planets. It would also explain the absence of clusters with
ages between 5 and 30Myr.
  We use the $\tau^2$ statistic to fit the main-sequence data, but find that we
must make significant modifications if we are to fit sequences which have
vertical segments in the colour-magnitude diagram. We present this modification
along with improvements to methods of calculating the goodness-of-fit statistic
and parameter uncertainties.
  Software implementing the methods described in this paper is available from
http://www.astro.ex.ac.uk/people/timn/tau-squared/
