This is not the case in other fields such as Engineering, Medicine, Education, or Law. More experienced engineers, doctors, lawyers, and educators tend to be better at their craft. Failing to keep up with new methodology/technology is also a problem.
It's almost like the quality of the person is more important than their age or seniority.
Now, I don't have any first-hand experience with legislatures. However, based on analogies in fields I'm more familiar with, I am a bit suspicious of the claim that there aren't any advantages to seniority and experience.
Also, consider this concrete question: would term limits fix the student debt problem? I don't see any arguments in this thread that it would.