How is race age calculated?

For most running events, your race age is determined to be your actual age on the day of the event.

However, for some events, including triathlons (and all USA Triathlon sanctioned events), your "race age" is determined to be the age you will be as of December 31 of the race year. So, for instance, let's say you turn 27 years old on December 30 and you're doing a USAT-sanctioned triathlon 6 months earlier, on June 15th. Your race age would actually be 27 years old even though you're only 26 at the time of the event.