More than just partisanship and ideology are on the line in today’s Indiana Senate primary. Also at stake is whether the Senate continues to grow older.

Congress, and the Senate in particular, have been creeping steadily older for some time, with each new Congress breaking records for age. That reversed a bit in the Republican landslide of 2010, with the average age in the Senate dropping from a record 63.1 down to 62.2 years old. Which way will the average move in 2013?

Currently, ten Senators are retiring, and Indiana’s Richard Lugar seems poised to lose today. Those eleven Senators will be an average of just under 72 years old in 2013. Their replacements? While it’s obviously still too early to know for sure, looking at the most likely new Senator in each of the eleven states gives an average age of just over 54 years. That’s a pretty large drop, just about enough to balance out the demographic momentum of the returning Senators all growing two years older.

What are the biggest differences? Today’s primary is certainly one of them. Richard Mourdock will be 61 in January, and Democrat Joe Donnelly will be 57. So if Lugar is defeated, that’s about a 20 year reduction, either way. The other one that looks fairly close now and has a large age spread is in Texas, where the Republicans are choosing between favorite David Dewhurst (67 in January) and Ted Cox (only 42). Others? In Connecticut, Chris Murphy (38) appears to have the edge over Linda McMahon (64) or perhaps Chris Shays (67); if Wisconsin is between Tammy Baldwin (50) and Jeff Fitzgerald (47) there’s not much difference, but if Fitzgerald loses the Republican primary to Tommy Thompson (71) it would set up a big age gap in the general election.

All of that leaves out the reasonably good chances that a candidate who looks weak now will emerge as a major player in one of these states, and it also leaves out the handful of states in which the incumbent is facing a tough contest in November.

So whatever else today’s Indiana primary means, a Lugar defeat would be a big step towards keeping the Senate younger. Which, individual Senators notwithstanding, is certainly something I’d like to see — indeed, I’d like to see the minimum age required to sit in the House or Senate lowered, as well. Again, there’s nothing wrong with any individual Senator being older, but put it all together, and it’s hard to believe that the United States is best served by a body in which as many look down at the Baby Boomers as look up to them.