Current President pro tempore of the Senate Robert Byrd is not only very old, but experienced serious health problems in recent years.
As by custom most senior Senator from majority party becomes President pro tem, which puts Daniel Inouye of Hawaii, 85, as first in the line. But the third most Senior Senator, a Democrat Pat Leahy of Vermont, is not yet 70. So I think he may have better shot than Inouye in next years.
If Republicans captures the majority, Lugar, sitting on safe seat and "not extremely old" for Senator, is first in line.
I know Senate pro tem is a low key post, but I'd like to raise this topic