  We have selected pre-main-sequence stars in 12 groups of notional ages
ranging from 1 Myr to 35 Myrs, using heterogeneous membership criteria. Using
these members we have constructed empirical isochrones in V, V-I colour
magnitude diagrams (CMDs). This allows us to identify clearly the gap between
the radiative main sequence and the convective pre-main-sequence (the R-C gap).
We follow the evolution of this gap with age and show that it can be a useful
age indicator for groups less than ~15 Myrs old. We also observe a reduction in
absolute spreads about the sequences with age. Finally the empirical isochrones
allow us to place the groups in order of age, independently of theory. The
youngest groups can be collated into three sets of similar ages. The youngest
set is the ONC, NGC6530 and IC5146 (nominally 1 Myrs); next Cep OB3b, NGC2362,
Lambda Ori and NGC2264 (nominally 3 Myrs); and finally Sigma Ori and IC348
(nominally 4-5 Myrs). This suggests Cep OB3b is younger than previously
thought, and IC348 older. For IC348 the stellar rotation rate distribution and
fraction of stars with discs imply a younger age than we derive. We suggest
this is because of the absence of O-stars in this cluster, whose winds and/or
ionising radiation may be an important factor in the removal of discs in other
clusters.
