D. Soderblom (Space Telescope Science Institute, Baltimore, United States),
L. Hillenbrand (Caltech, Pasadena, United States),
R. Jeffries (Astrophysics Group, Keele University, Staffordshire, United Kingdom),
E. Mamajek (Dept. of Physics & Astronomy, University of Rochester, United States),
T. Naylor (School of Physics, University of Exeter, United Kingdom)

Determining the sequence of events in the formation of stars and planetary systems and their timescales is essential for understanding these processes, yet establishing ages is fundamentally difficult because we lack direct indicators. In this review we discuss the problem of ages for young stars, specifically for those less than ~ 100 Myr old. We start by establishing a reliable scale scale for young stars using the measurement of the Lithium Depletion Boundary (LDB) in young clusters, a method that involves fairly simple and wellestablished physics and observations. We show that LDB ages are consistent with those from the upper main sequence and the main sequence turn-off if modest core convection is included in the models of higher-mass stars. The LDB method can be used to set limits on the age of individual objects but is primarily applicable to groups of stars. We then review the available methods for age estimation, which include kinematic traceback of young groups, placing stars in HRDs, pulsations and seismology, gravity differences, rotation, activity, and lithium abundance as age indicators. We list the known strengths and weaknesses of each of these. Some of these techniques are useful within certain limits (such as age and mass ranges) and others are entirely problematic. We look at the issue of age spreads within star-forming regions and how well they can be quantified. Finally, we offer suggestions and recommendations to guide interpretation of observations, and we show how the current situation can be improved over the next few years.

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