Variation of the linear size and the separation velocity with kinematic age. (a) Both source groups (high and low radio power) are consistent with hotspot separation velocity decreasing with age. The predicted evolution of the sources follows a v _HS∝ T ^−2/5 _kin relation (dashed arrows). (b) The CSO linear size shows an expected increase with age. The predicted variation for individual sources with constant power is D∝ T ^3/5 _kin as indicated with dashed arrows. As a group the older sources also have larger sizes as D∝ T ^1.5 _kin (dotted line).