I'm a Junior (17) and a Cat 2.
Cat 5 races might average aroud 21 - 25 mph: Speed is dependant on whether the field is large or small, and if the course is hilly or not.
Junior races unflod unlike any categorized race. Although I'd say that the average junior in the 15 - 18 age group is a Cat 4, the levels of strength vary dramatically, from Cat 5 to Cat 2 (there were two Cat 1 juniors in the country last year, but as far as I know, they never did junior races).
Since a lot of the time fields are very small (less than 25) in Junior races, and most juniors don't like to do pulls at the front, the speed fluctuates DRAMATICALLY from 17 mph (when nobody feels like attacking), to brief periods at 30 mph (when the testosterone gets flowing and EVERYONE is attacking).
The average speed of a junior race is NEVER reflective of how difficult it actually was. Average speeds can be as low as 20 mph for a hillyish course. Very low speeds during portions of the race drag the average speeds way down. This phenomenon of fluctuating speeds in Junior races is not present however at the "big" junior races like Nationals and other times when the field is over 40 riders.
Assuming your a junior getting into racing, I would recommend (if possible) to do both the Junior Race AND the Categorized Race at a given venue. You'll improve much faster with all the racing miles, and upgrade faster too (it should be noted that Junior races count towards upgrade points untill you become a Cat 3).