  We present a spectroscopic study and dynamical analysis of ~2600 M7 dwarfs.
We confirm our previous finding that the fraction of magnetically active stars
decreases with vertical distance from the Galactic plane. We also show that the
mean luminosity of the H-alpha emission has a small but statistically
significant decrease with distance. Using space motions for ~1300 stars and a
simple one-dimensional dynamical simulation, we demonstrate that the drop in
the activity fraction of M7 dwarfs can be explained by thin disk dynamical
heating and a rapid decrease of magnetic activity at a mean stellar age of ~6-7
Gyr.
