  Before the launch of the Kepler Space Telescope, models of low-mass planet
formation predicted that convergent Type I migration would often produce
systems of low-mass planets in low-order mean-motion resonances. Instead,
Kepler discovered that systems of small planets frequently have period ratios
larger than those associated with mean-motion resonances and rarely have period
ratios smaller than those associated with mean-motion resonances. Both
short-timescale processes related to the formation or early evolution of
planetary systems and long-timescale secular processes have been proposed as
explanations for these observations. Using a thin disk stellar population's
Galactic velocity dispersion as a relative age proxy, we find that
Kepler-discovered multiple-planet systems with at least one planet pair near a
period ratio suggestive of a second-order mean-motion resonance have a colder
Galactic velocity dispersion and are therefore younger than both
single-transiting and multiple-planet systems that lack planet pairs consistent
with mean-motion resonances. We argue that a non-tidal secular process with a
characteristic timescale no less than a few hundred Myr is responsible for
moving systems of low-mass planets away from second-order mean-motion
resonances. Among systems with at least one planet pair near a period ratio
suggestive of a first-order mean-motion resonance, only the population of
systems likely affected by tidal dissipation inside their innermost planets has
a small Galactic velocity dispersion and is therefore young. We predict that
period ratios suggestive of mean-motion resonances are more common in young
systems with 10 Myr $\lesssim \tau \lesssim 100$ Myr and become less common as
planetary systems age.
