Globular cluster age estimates based on the absolute magnitude of the main sequence turn-off ( { M _ { v } ( TO ) } ) are generally considered to be the most reliable from a theoretical viewpoint .
However , the difficulty in determining { M _ { v } ( TO ) } in observed colour-magnitude diagrams leads to a large error in the derived age .
In this paper , we advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder ( { M _ { v } ( BTO ) } ) as a precision age indicator .
It is easy to measure this point on observed colour-magnitude diagrams , leading to small observational error bars .
Furthermore , an extensive Monte Carlo calculation indicates that the theoretical uncertainty in { M _ { v } ( BTO ) } is similar to { M _ { v } ( TO ) } .
As a result , ages derived using { M _ { v } ( BTO ) } are at least a factor of 2 more precise than those derived using { M _ { v } ( TO ) } .
This technique is applied to the globular cluster M68 and an age of 12.8 \pm 0.3 Gyr is derived ( assuming \hbox { $ { M _ { v } ( RR ) } $ } = 0.20 \hbox { $ [ { Fe } / { H } ] $ } +0.98 ) , indicating that M68 is a ‘ young ’ globular cluster .
A homogeneous set of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo .