  Infrared (IR) spectral energy distribution (SED) is the major tracer of
protoplanetary disks. It was recently proposed to use the near-to-mid IR (or
K-24) SED slope $\alpha$ defined between 2-24$\mu$m as a potential quantitative
tracer of disk age. We critically examine the viability of this idea and
confront it with additional statistics of IR luminosities and SED shapes. We
point out that, because the statistical properties of most of the complicated
physical factors involved in disk evolution are still poorly understood in a
quantitative sense, the only viable way is to assume them to be random so that
an idealized `average disk' can be defined, which allows the $\alpha$ histogram
to trace its age. We confirm that the statistics of the zeroth order
(luminosity), first order (slope $\alpha$) and second order characteristics
(concavity) of the observed K-24 SEDs indeed carry useful information upon the
evolutionary processes of the `average disk'. We also stress that intrinsic
diversities in K-24 SED shapes and luminosities are always large at the level
of individual stars so that the application of the evolutionary path of the
`average disk' to individual stars must be done with care. The data of most
curves in plots are provided on GitHub.
