[emanuelev]

I'm sorry, I don't really have a source. My observation is based on the fact that during PhD/PostDoc years you have more time to spend on actual research. As you climb the academic ladder it is very probable that administrative/supervising/teaching duties become central to your job (unless you really don't want to).

Incidentally I remember also reading [1] that in certain fields, in this case mathematics, most of the groundbreaking research comes from younger mathematicians. Great contributions to the field from people over 40 are extremely rare.

[1] Simon Singh, Fermat's Last theorem. Ok, not great source but still!