  M dwarf stars are not only the most promising hosts for detection and
characterization of small and potentially habitable planets, they provide
leverage relative to solar-type stars to test models of planet formation and
evolution. Using Gaia astrometry, adaptive optics imaging, and calibrated
gyrochronologic relations to estimate stellar properties, filter binaries, and
assign ages, we refined the radii of 179 transiting planets orbiting 119 single
late K- and early M-type stars detected by the Kepler mission, and assigned
stellar rotation-based ages ) to 115 of these. We constructed the radius
distribution of <4R$_{\oplus}$ planets and assessed its evolution with time. As
for solar-type stars, the inferred distribution contains distinct populations
of "super-Earths" (at ~1.3R$_{\oplus}$) and "sub-Neptunes" (at ~2.2Rearth)
separated by a gap or "valley" at $\approx$1.7R$_{\oplus}$ that has a period
dependence that is significantly weaker (power law index of
-0.026$^{+0.026}_{-0.017}$) than for solar-type stars. Sub-Neptunes are largely
absent at short periods ($<$2 days) and high irradiance, a feature analogous to
the "Neptune desert" observed around solar-type stars. The relative number of
sub-Neptunes to super-Earths declines between the younger and older halves of
the sample (median age 3.8 Gyr), although the formal significance is low ($p =
0.06$) because of the small sample size. The decline in sub-Neptunes appears to
be more pronounced at long orbital periods vs. short periods; this is not due
to detection bias and could indicate that these objects are inflated by a
mechanism that operates at elevated irradiance, e.g. a runaway water greenhouse
augmented by H/He.
