Ambipolar-Diffusion Timescale, Star-Formation Timescale, and the Ages of   Molecular Clouds: Is There a Discrepancy?
  We re-examine critically the estimates of the duration of different phases of star formation and the lifetimes of molecular clouds, based on the ages of T-Tauri stars, age spreads of stars in clusters, and statistics of pre-stellar cores. We show that all available observational data are consistent with lifetimes of molecular clouds comparable to 10 Myr, as well as with the predictions of the theory of self-initiated, ambipolar-diffusion--controlled star formation. We conclude that there exists no observational support for either "young" molecular clouds or "rapid" star formation. 
Konstantinos Tassis and Telemachos Ch. Mouschovias (Departments of   Physics and Astronomy, University of Illinois at Urbana...