  Gyrochronology is one of the methods currently used to estimate the age of
stellar open clusters. Hundreds of new clusters, associations, and moving
groups unveiled by Gaia and complemented by accurate rotation period
measurements provided by recent space missions such as Kepler and TESS are
allowing us to significantly improve the reliability of this method. We use
gyrochronology, that is, the calibrated age-mass-rotation relation valid for
low-mass stars, to measure the age of the recently discovered moving group
Group X. We extracted the light curves of all candidate members from the TESS
full frame images and measured their rotation periods using different period
search methods. We measured the rotation period of 168 of a total of 218 stars
and compared their period-colour distribution with those of two age-benchmark
clusters, the Pleiades (125 Myr) and Praesepe (625 Myr), as well as with the
recently characterised open cluster NGC3532 (300 Myr). As result of our
analysis, we derived a gyro age of 300$\pm$60 Myr. We also applied as
independent methods the fitting of the entire isochrone and of the three
brightest candidate members individually with the most precise stellar
parameters, deriving comparable values of 250 Myr and 290 Myr, respectively.
Our dating of Group X allows us to definitively rule out the previously
proposed connection with the nearby but much older Coma Berenices cluster.
