One way to facilitate comparisons between academics with different lengths of academic careers is to divide the h-index by the number of years the academic has been active (measured as the number of years since the first published paper). Hirsch (2005) proposed this measure and called it m.
However, we should note that m generally does not stabilise until later in ones career and that for junior researchers (with low h-indices) small changes in the h-index can lead to large changes in m. In addition, as Hirsch indicates, the first paper may not always be the appropriate starting point, especially if it was a minor contribution that was published well before the academic realised a sustained productivity. Moreover, m discriminates against academics that work part-time or have had career interruptions (generally women). However, in some cases m might be a useful additional metric to evaluate an academics achievement.