  We study the ensemble X-ray variability properties of Active Galactic Nuclei
(AGN) over a large range of timescales (20 ks $\leq T\leq$ 14 yrs), redshift
($0\leq z \lesssim 3$), luminosities ($10^{40}$ erg s$^{-1}\leq L_X\leq
10^{46}$ erg s$^{-1}$) and black hole (BH) masses ($10^6 \leq $M$_\odot \leq
10^9$). We propose the use of the variance-frequency diagram, as a viable
alternative to the study of the power spectral density (PSD), which is not yet
accessible for distant, faint and/or sparsely sampled AGN. We show that the
data collected from archival observations and previous literature studies are
fully consistent with a universal PSD form which does not show any evidence for
systematic evolution of shape or amplitude with redshift or luminosity, even if
there may be differences between individual AGN at a given redshift or
luminosity. We find new evidence that the PSD bend frequency depends on BH mass
and, possibly, on accretion rate. We finally discuss the implications for
current and future AGN population and cosmological studies.
