  We investigate how the observed large-scale surface magnetic fields of
low-mass stars (~0.1 -- 2 Msun), reconstructed through Zeeman-Doppler imaging
(ZDI), vary with age t, rotation and X-ray emission. Our sample consists of 104
magnetic maps of 73 stars, from accreting pre-main sequence to main-sequence
objects (1 Myr < t < 10 Gyr). For non-accreting dwarfs we empirically find that
the unsigned average large-scale surface field <|Bv|> is related to age as
$t^{-0.655 \pm 0.045}$. This relation has a similar dependence to that
identified by Skumanich (1972), used as the basis for gyrochronology. Likewise,
our relation could be used as an age-dating method ("magnetochronology"). The
trends with rotation we find for the large-scale stellar magnetism are
consistent with the trends found from Zeeman broadening measurements (sensitive
to large- and small-scale fields). These similarities indicate that the fields
recovered from both techniques are coupled to each other, suggesting that
small- and large-scale fields could share the same dynamo field generation
processes. For the accreting objects, fewer statistically significant relations
are found, with one being a correlation between the unsigned magnetic flux and
rotation period. We attribute this to a signature of star-disc interaction,
rather than being driven by the dynamo.
