Recently I encountered a new metric, the "5 year h-index" which was being used to evaluate someone's research performance. Explicity, one counts the number h of papers published in the last 5 years that have been cited more than h times. Perhaps one might argue that this is a good metric for deciding whether to give someone a grant now. Afterall, just because 10 or 20 years ago they published highly cited papers does not mean that right now they are at the cutting edge. However, I do not agree.
I think this is a highly unreliable metric because there is significant noise. Except for a few rare exceptional papers, citations within a few years of publication will be low (1-10?). Hence, comparing two people with 5 year h-indexes, say one with a 6 and another with a 10, I would contend is meaningless.
Two of the most cited papers in Physical Review journals are the EPR (Einstein Podolsky Rosen) paper and Steven Weinberg's electro-weak interaction paper. The latter attracted about one citation per year for the first 5 years after publication! The former attracted about 10 citations in the first 2 years and then none for more than ten years!
When Jorge Hirsch introduced the h-index the whole point what to find some measure for a lifetime of scientific achievement. I still think it does provide a useful coarse-grained measure for that.