I have never used octal numbers in my code nor come across any code that used it. I bet you have. According to the standard, numeric literals which start with zero are octal. This includes, trivially, 0. Every time you have used or seen a literal zero, this has been octal. Strange but true.
If you are making child support payments in Texas, the law generally requires you to do so until your child turns 18, which is the age that marks adulthood in the state, or when he graduates from high school -- whichever comes later.
Another reason for child support to stop is the age of the child. If one child has turned 18 (and it is July of the year the child finished high school), but there are other children under 18, there is an automatic reduction in the amount of support, based on the % reduction in the number of children.