  We study the statistics of citations from all Physical Review journals for
the 110-year period 1893 until 2003. In addition to characterizing the citation
distribution and identifying publications with the highest citation impact, we
investigate how citations evolve with time. There is a positive correlation
between the number of citations to a paper and the average age of citations.
Citations from a publication have an exponentially decaying age distribution;
that is, old papers tend to not get cited. In contrast, the citations to a
publication are consistent with a power-law age distribution, with an exponent
close to -1 over a time range of 2 -- 20 years. We also identify a number of
strongly-correlated citation bursts and other dramatic features in the time
history of citations to individual publications.
