At the Arkansas Super-Prestige, we've been a little loosey-goosey about racer age in the past. But the USA Cycling rules are unequivocal:
A rider’s racing age in cyclo-cross is his age on December 31st of the year that the cyclo-cross season ends. (Glossary, USAC Rulebook)
That means racer age for the 2014 Arkansas Super-Prestige is age as of December 31, 2015.