Junior year is the year most people turn 21.
If you do a good job of manipulating your sched. You can take tougher classes as a junior and have a pud senior year like I did. I took 17 and 18 hours both semesters my junior year, most people I knew did the same.
A lot of "senior" level classes are pretty easy. I think they figure that they put you through so much crap as an underclassmen that they take it easy on you at the end.