BRING YOUR LICENSE TO THE RACE. They will be checking that you are indeed entering the correct race(s). One-day licensees have this one covered.
Your USAC cyclocross category is printed on your license:
|A visual aid|
If you are racing on a one-day license, then you are CX:05 if you are male or CX:04 if you are female (there are only 4 CX categories for women). If you want to change your category, you must make a request to usacycling.org. This is not something that can be performed on race day.
OK here's where it's a little tricky. Because the national cyclocross calendar extends into the new year, your cyclocross racing age is how old you will be on December 31, 2015.
USAC rule §1I1(i) is pretty straightforward:
A Single Speed is any type of bicycle possessing only one rear cog and only one front chainring and with no means of altering the gear ratio in any way during the race.
To summarize, BRING YOUR LICENSE. If you can't find it, print off a copy by logging on to usacycling.org. Do it now and put it in your race bag. [We have also been informed that USAC has a mobile app that allows you to show your license on your electronic device. Yet another option.]