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We revisit the problem of low-mass pre-main-sequence (PMS) stellar evolutionand its observational consequences for where stars fall on theHertzsprung-Russell diagram (HRD). In contrast to most previous work, ourmodels follow stars as they grow from small masses via accretion, and weperform a systematic study of how the stars' HRD evolution is influenced bytheir initial radius, by the radiative properties of the accretion flow, and bythe accretion history, using both simple idealized accretion histories andhistories taken from numerical simulations of star cluster formation. Wecompare our numerical results to both non-accreting isochrones and to thepositions of observed stars in the HRD, with a goal of determining whether boththe absolute ages and the age dispersions inferred from non-accretingisochrones are reliable. We show that non-accreting isochrones can sometimesoverestimate stellar ages for more massive stars (those with effectivetemperatures above \sim 3500 K), thereby explaining why non-accretingisochrones often suggest a systematic age difference between more and lessmassive stars in the same cluster. However, we also find the only way toproduce a similar overestimate for the ages of cooler stars is by selectingparameters that are strongly inconsistent with both theoretical models andobservational constraints. We therefore conclude that inferred ages and agespreads for cool stars are reliable, at least to the extent that the observedbolometric luminosities and temperatures are accurate. Finally, we note thatthe time-dependence of the mass accretion rate has remarkably little effect onlow-mass stars' evolution on the HRD, and that such time-dependence may beneglected for all stars except those with effective temperatures above \sim4000 K.

On the Reliability of Stellar Ages and Age Spreads Inferred from Pre-Main Sequence Evolutionary Models