  Main sequence turn-off (MSTO) stars are good tracers of Galactic populations
since their ages can be reliably estimated from atmospheric parameters. Based
on the GALAH survey, we use the Yale Rotation Evolution Code to determine ages
of 2926 MSTO stars with mean age uncertainty $\sim$10\% considering the
variation of C and O abundances. Ages of CO-poor stars are systematically
affected by $\sim$10\% due to the C and O abundances, globally shifting to
$\sim$0.5 Gyr older compared to the results using solar metal-mixture. Of the
stars with \mbox{[Fe/H] $\sim$0.3-0.5} or \mbox{[O/Fe]~$\leq$~-0.25}, many have
fractional age differences~$\geq$~20\%, and even reach up to 36\%. The
age-metallicity relation appears to possibly exist two distinct sequences: a
young sequence of stars with age mostly $<$ 7 Gyr, and a relatively older
sequence of stars with age mostly $>$ 7 Gyr, overlapping at 5 Gyr
$\leq$~age~$\leq$ 7 Gyr. Moreover, the trends of abundances to age ratios show
two corresponding sequences, especially in [O/Fe]-age plane. We also find that
[Y/Mg] is a good chemical clock in disk populations. The young sequence and the
old sequence can not be separated based on chemistry or kinematics, therefore
stellar age is an important parameter to distinguish these two sequences in our
sample.
