  Oxygen and carbon are important elements in stellar populations. Their
behavior refers to the formation history of the stellar populations. C and O
abundances would also obviously influence stellar opacities and the overall
metal abundance $Z$. With observed high-quality spectroscopic properties, we
construct stellar models with C and O elements, to give more accurate ages for
70 metal-poor dwarfs, which have been determined to be high-$\alpha$ halo,
low-$\alpha$ halo and thick-disk stars. Our results show that high-$\alpha$
halo stars are relatively older than low-$\alpha$ halo stars by around 2.0 Gyr.
The thick-disk population has an age range between the two halo populations.
The age distribution profiles indicate that high-$\alpha$ halo and low-$\alpha$
halo stars match the in situ accretion simulation by Zolotov et al., and the
thick-disk stars might be formed in a relatively quiescent and long-lasting
process. We also note that stellar ages are very sensitive to O abundance,
since the ages clearly increase with increasing [O/Fe] values. Additionally, we
obtain several stars with peculiar ages, including 2 young thick-disk stars and
12 stars older than the universe age.
