Main sequence turnoff magnitudes from the recent set of Yale isochrones ( Chaboyer et al . 1995 ) have been combined with a variety of relations for the absolute magnitude of RR Lyr stars ( M _ { v } ( RR ) ) to calibrate age as a function of the difference in magnitude between the main sequence turn-off and the horizontal branch ( \Delta V ^ { TO } _ { HB } ) .
A best estimate for the calibration of M _ { v } ( RR ) is derived from a survey of the current literature : \hbox { $ M _ { v } ( RR ) $ } = 0.20 \hbox { $ [ { Fe } / { H } ] $ } +0.98 .
This estimate , together with other calibrations ( with slopes ranging from 0.15 to 0.30 ) has been used to derive \Delta V ^ { TO } _ { HB } ages for 43 Galactic globular clusters .
Independent of the choice of M _ { v } ( RR ) , there is no strong evidence for an age-Galactocentric distance relationship among the 43 globular clusters .
However , an age-metallicity relation exists , with the metal-poor clusters being the oldest .
A study of the age distribution reveals that an age range of 5 Gyr exists among the bulk of the globular clusters .
In addition , about 10 % of the sample are substantially younger , and including them in the analysis increases the age range to 9 Gyr .
Once again , these statements are independent of the M _ { v } ( RR ) relation .
Evidence for age being the second parameter governing horizontal branch morphology is found by comparing the average \Delta V ^ { TO } _ { HB } age of the second parameter clusters to the normal clusters .
The second parameter clusters are found to be on average 2 – 3 Gyr younger than the other clusters , which is consistent with age being the second parameter .
These results suggest that globular clusters were formed over an extended period of time , with progressively more metal-rich globular clusters ( \hbox { $ [ { Fe } / { H } ] $ } \ga - 1.7 ) being formed at later times .