Turn off point

As stars evolved off the main sequence they became the giants (on the upper right), and then the white dwarfs (on the lower left). Identify the turn off point along the main sequence. This is the value of $M$ and $B-V$ where stars in M67 are now turning off the main sequence to move into the giant branch. The turn off point depends on the age (and metallicity) of the cluster.

Approximately the main sequence lifetime of a star in years is

$$t = 10^{10} \left(\frac{L}{L_{Sun}}\right)^{-3}$$

where L is the luminosity of the star. Thus a star like the Sun has a lifetime of $10^{10}$ years, but one 10 times more luminous will live only $10^7$ years on the main sequence. The Sun has an absolute magnitude of +4.8 and a color index (B-V) of +0.65.

Approximately how old is M67?