We have derived ages for 13 young ( < 30 Myr ) star-forming regions and find they are up to a factor two older than the ages typically adopted in the literature .
This result has wide-ranging implications , including that circumstellar discs survive longer ( \simeq 10 - 12 Myr ) and that the average Class I lifetime is greater ( \simeq 1 Myr ) than currently believed .

For each star-forming region we derived two ages from colour-magnitude diagrams .
First we fitted models of the evolution between the zero-age main-sequence and terminal-age main-sequence to derive a homogeneous set of main-sequence ages , distances and reddenings with statistically meaningful uncertainties .
Our second age for each star-forming region was derived by fitting pre-main-sequence stars to new semi-empirical model isochrones .
For the first time ( for a set of clusters younger than 50 Myr ) we find broad agreement between these two ages , and since these are derived from two distinct mass regimes that rely on different aspects of stellar physics , it gives us confidence in the new age scale .
This agreement is largely due to our adoption of empirical colour- T _ { eff } relations and bolometric corrections for pre-main-sequence stars cooler than 4000 K .

The revised ages for the star-forming regions in our sample are – \sim 2 Myr for NGC 6611 ( Eagle Nebula ; M 16 ) , IC 5146 ( Cocoon Nebula ) , NGC 6530 ( Lagoon Nebula ; M 8 ) , and NGC 2244 ( Rosette Nebula ) ; \sim 6 Myr for \sigma Ori , Cep OB3b , and IC 348 ; \simeq 10 Myr for \lambda Ori ( Collinder 69 ) ; \simeq 11 Myr for NGC 2169 ; \simeq 12 Myr for NGC 2362 ; \simeq 13 Myr for NGC 7160 ; \simeq 14 Myr for \chi Per ( NGC 884 ) ; and \simeq 20 Myr for NGC 1960 ( M 36 ) .