The birthplace and age of the isolated neutron star RX J1856.5-3754

X-ray observations unveiled various types of radio-silent isolated neutron stars (INSs), phenomenologically very diverse, e.g. the similar to Myr old X-ray-dim INSs (XDINSs) and the similar to kyr old magnetars. Although their phenomenology is much diverse, the similar periods (P = 2-10 s) and magnetic fields (approximate to 10(14) G) suggest that XDINSs are evolved magnetars, possibly born from similar populations of supermassive stars. One way to test this hypothesis is to identify their parental star clusters by extrapolating backwards the NS velocity vector in the Galactic potential. By using the information on the age and space velocity of the XDINS RXJ1856.5-3754, we computed backwards its orbit in the Galactic potential and searched for its parental stellar cluster by means of a closest approach criterion. We found a very likely association with the Upper Scorpius OB association, for a NS age of 0.42 +/- 0.08 Myr, a radial velocity V-r(NS) = 67 +/- 13 km s(-1), and a present-time parallactic distance d(pi)(NS) = 123(-15)(+11) pc. Our result confirms that the 'true' NS age is much lower than the spin-down age (tau(sd) = 3.8 Myr), and is in good agreement with the cooling age, as computed within standard cooling scenarios. The mismatch between the spin-down and the dynamical/cooling age would require either an anomalously large breaking index (n similar to 20) or a decaying magnetic field with initial value B-0 approximate to 10(14) G. Unfortunately, owing to the uncertainty on the age of the Upper Scorpius OB association and the masses of its members, we cannot yet draw firm conclusions on the estimated mass of the RXJ1856.5-3754 progenitor.