There are two ways to race, catagory or age class.
I.e.
Cat 5, Cat 4, Cat 3, Cat 2, Cat 1, Pro
or
Juniors, Seniors 18-29 (actully any age over 18), Masters 30+, 40+, 45+, 55+, 65+ ,etc. The most common are 30+ and 40+.
Sometimes you will find Cat 4/5 40+ types races also.
The masters races that are not catagory based ^^^ can be wickedly fast but quite smooth, the normal senior level races can also be fast but are generally hairier (more crash prone).
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Just your average club rider... :)