  The position of pre-main-sequence or protostars in the Hertzsprung--Russell
diagram is often used to determine their mass and age by comparison with
pre-main-sequence evolution tracks. On the assumption that the stellar models
are accurate, we demonstrate that, if the metallicity is known, the mass
obtained is a good estimate. However, the age determination can be very
misleading because it is significantly (generally different by a factor of two
to five) dependent on the accretion rate and, for ages less than about one
million years, the initial state of the star. We present a number of accreting
protostellar tracks that can be used to determine age if the initial conditions
can be determined and the underlying accretion rate has been constant in the
past. Because of the balance established between the Kelvin-Helmholtz,
contraction timescale and the accretion timescale a pre-main-sequence star
remembers its accretion history. Knowledge of the current accretion rate,
together with an H--R-diagram position gives information about the rate of
accretion in the past but does not necessarily improve any age estimate. We do
not claim that ages obtained by comparison with these particular accreting
tracks are likely to be any more reliable than those from comparisons with
non-accreting tracks. Instead we stress the unreliability of any such
comparisons and use the disparities between various tracks to estimate the
likely errors in age and mass estimates. We also show how a set of coeval
accreting objects do not appear coeval when compared with non-accreting tracks.
Instead accreting pre-main-sequence stars of around a solar mass are likely to
appear older than those of either smaller or larger mass.

Astrophysics