We use the age the competitor is on the day of the tournament. So if your child is 13 years old on the day of they're tournament they'll fight in the 12-13 year old division, even if they turn 14 the next day, week, month. Kids may always fight in an older age bracket if they so choose for more of a challenge.
If an adult is 29 on the day of the tournament they must compete in Adult, even if they're turning 30 later in the year.