Okla. Stat. tit. 15, § 13
Minors, except as otherwise provided by law, are persons under eighteen (18) years of age.
The period thus specified must be calculated from the first minute of the day on which a person is born to the same minute of the corresponding day completing the period of minority.
R.L.1910, § 879; Amended by Laws 1972, SB 515, c. 221, § 1.