U.S. Senator tenure
January 16, 2010
The chart below shows the maximum (solid line), average (circles) and median (horizontal bars) of the distribution of tenures of U.S. senators serving at each year since the establishment of the body.
Over the last 200 years all three indicators of tenure length have been increasing steadily, approximately doubling every 100 years. There are now 52 senators with tenure of ten years or more, and two senators, Robert Byrd and Daniel Inouye, with tenure of about 50 years.
The chart was created using a table that I prepared based on the chronological list of senators available on the senate website. There may be some slight inaccuracies in the table, but I believe that on the whole the data is valid.
Pro Bono Statistics 
January 20, 2010 at 4:13 am
You are comparing Apples and Oranges, at least for the time before 1912.
Before 1914, Senators were selected by the state legislatures, after 1914, they were elected.
See the Wiki on the 17th amendment.
I am not sure how you would correct for the change, and apart from the 1914 notch, it appears to track fairly consistently.
Some possibilities as to the reasons for the trends:
Ease of travel makes it possible for older people to serve.
Seniority has always been a source of power in the Senate, and it pushes the trend.
Additional benefits to incumbency?
How does this compare to House elections?
January 20, 2010 at 6:07 am
Comparing things over 200 years is always a risky business – that is why I did not offer an interpretation of the data in the post. Lots of things changed in the U.S. over the last two centuries. For one thing, Senate used to have only 26 members. It is actually quite surprising then how steady was the increase in Senator tenure. One could expect, for example, something to happen in 1914 – yet no change in the pattern is apparent. By the way, as Wikipedia mentions, the transition to directly elected Senators was not abrupt – by 1912, 29 states had some form of direct elections for Senators.
Since you ask, however, here is my interpretation: to me, the lengthening tenure indicates the increasing consolidation of power in the US. As financial and political power become more concentrated and converge, the elite becomes more stable. This, of course, is somewhat speculative. Naturally, increase in life expectancy probably plays some role as well.
Examining the trend in House tenures would indeed be interesting. I doubt that the picture would be different.