The President Pro Temp, or PPT, is even part of the presidential line of succession. It's switched positions many times with the Speaker of the House of Representatives and the Cabinet Secretaries, but since 1947 it has remained 3rd in line behind the VPOTUS and the Speaker and just before the Secretary of State.
Starting in 1890 the Senate started electing the senior senator of the majority party to the office and since 1949 has stuck to that tradition. It's a strange tradition to reenforce when the presidential line of succession was just rewritten two years prior and it was the early years of the Cold War. It would take an assassination and a vacancy in the vice presidency where a Constitutional Amendment would be needed and so came the 25th Amendment in 1967. One of the four sections of the Amendment state that the president must appoint a vice president if the office were vacant. The line of succession during the height of the Cold War in the 1960s was something of importance in maintaining the continuity of government.
The United States faced similar situations 2002 when the government was reorganized again with the creation of the Department of Homeland Security. However while the line of succession was always considered during the early years of the "War on Terror", the Senate didn't break with tradition when it came to electing their leadership and thus third person in line for the presidency was an old man who's age was showing.
Yes, the line of succession has never gone beyond the vice president but since it is there it should be taken seriously. The President Pro Tempore should become the Senate's power equivalent to that of the Speaker. (And yes, the Speaker is equal to the President of the Senate as leaders of the 2 houses). The PPT should earn the pay check and have more of a role...even though they already don't do anything. The VPOTUS and the PPT hardly ever preside over Senate session. That's what freshmen are for...at any institution. The Senate leadership of President Pro Temp, Majority Leader and Majority Whip (aka Assistant Majority Leader) should mirror the leadership of the House with the Speaker, Majority Leader and so on.
The current PPT is Patrick Leahy of Vermont and at 73 years old doesn't look his age like his most recent predecessors like Daniel Inouye of HI, Robert Byrd of WV, Ted Stevens of AK, and Strom Thurmond of SC. Now some perspective, when Senator Byrd took the office of PPT for the first time in 1989 he was 72 and as the power shifted back-and-forth between Republicans and Democrats so did the occupant of the Office of the President Pro Tempore, so by 2010 (the year Byrd passed away) he was 93 years old when he last held the office. As our recent PPT have shown, humans are living longer and without term limits (not that GIXING FOVERNMENT is going to to mention term limits as something that will remedy the system...but it will be touched on), Senator Leahy will be the PPT each time the Democrats are in power. If Leahy continues to serve, then 20 years later (2034), we'll see an older version of Leahy as the PPT.
If a man looked as old as any of Leahy's most recent predecessors, no American would vote for him (or her). Age at some point becomes a factor in the consideration for the presidency. So far, Ronald Reagan is the oldest POTUS at 69. For 8 years he served and we did not question his ability to serve, even though 5 years after leaving office he revealed he had Alzheimer's. Vice President Alben Barkley was advised not to run for president in 1952 because, at 74, his age would be a factor. In 2014, 74 doesn't look the same to Americans as it did in the mid-20th Century. Currently, the average age of a POTUS is 54 years old, or rounding up, 55. So, the age of the occupant of the Office of the President Pro Temp should be reasonable and not seem like they are challenging the limits of humanity.
While this smacks of ageism, we already Constitutionally limit the office holder of the presidency to 35 and above, as well as being a native born citizen. Those younger than 35 are deemed unqualified for the office. If there is a non-native born member of the Cabinet, they would be skipped in the line of succession because they don't meet the qualifications for the office. A Constitutional Amendment is not needed for limiting the age of the PPT, but rather the Senate simply reverting to the pre-1890 practices of simply voting for the officer. If the Senate treats the selection of the PPT like that of the Speaker, then the result will more than likely be a politician in their political prime...and yes that might be an aging person but not likely. So let's stop giving a line of succession job to longest serving member of a party, just create a special office for that person and leave the Constitutional office to those that could serve all Americans if need be.