YATES

I think i'm entering my first race.. The Mississippi Gran Prix.. I haven't registered with USCF yet. As a 19 year old, will i be classified as a Cat 5 senior? or a junior.. and also what is a "master"?

DannoXYZ 

Juniors are under 18 and race their own race with gearing-restrictions. Junior races are similar to cat-3/4 in speed. Masters are typically divided into age groups: 35+, 45+, etc. This is one way of splitting up the large entries of cat-4 and cat-5 riders so that you don't have 250+ people in the cat-4 race. But be aware that the Masters races are full of seasoned vets and their races are just slightly under the cat-1/2/pro race in speed & viciousness. Personally, I'd like to see categories based strictly on ability and nothing to do with age. So we can have a race grouped as follows:
That should provide even field-sizes for 400-600 riders/weekend. So for you, you'd be entered in the cat-5/public race.

Duke of Kent

In my experience, the juniors races are nowhere near as fast as the Cat 3 races. Most of them got blown out the back of the 4s races last year. The few juniors that ARE that fast are the ones who ARE Cat 2s or3s by merit of racing both juniors and then 4s races and getting upgrade points.

vpiuva

As a 19 year old you are classified as a U-23 (your current racing age is your age on Dec 31, 2007, so either 19 or 20) . You are not subject to junior gearing restrictions, so no worry about that. You will have to enter as a Cat 5, and unless your race has a U-23 class you will be racing in the general (also known as elite) class. At any event you will be allowed to ride in either or both the U-23 and elite class races, but not youth, juniors, or any masters class. You can buy your license beforehand here https://www.usacycling.org/ or at the event. Good luck and have fun