If you're under 18 - Juniors.
18-30 - Elite
30+ Masters

Within all of this you have Categories that supposedly roughly describe your fitness and ability. In men's there is - from low to high - Cat V-I (5 through 1). You must progress through the ranks of these categories by meeting criteria.

From 5 to 4 for example you need to participate in 10 mass start events.

You can race within Cat in both Elite and Masters if you meet the age criteria.

Example. 38 yr old Cat 4 racer can usually participate in the Elite 4 race as well as the masters race of their age group or category. (Maters 35+ or Masters 4-5)

Juniors can do the same (Juniors 16-17, Cat 4 "Elite")

This only begins to describe it. It does seem complicated, but it really isn't. If you're over 18 and under 30 then you're stuck in Elite. Just starting out you would only be able to race Cat 5. So the only races you could ener would be ones that sound like one of the following:

Cat V (5)
Cat IV-V (4-5)
Elite 5
Elite 4-5

Citizens is the same except without licenses. You need a license for anything.