  We propose a new approach in determining ages of FR II type radio sources. We
apply the assumed dynamical model of Kaiser et al. (1997) to a number of FR II
type radio galaxies observed at different radio frequencies, and fit - for each
frequency separately - the model free parameters to the observed sources'
quantities. Such a procedure, using enlarged in fact a number of observables,
enables us to determine relatively precise ages and other crucial
characteristics of the analyzed sources. The resulting age estimates agree very
well with those obtained by means of `classical' spectral ageing method for
objects not older than 10 Myr, for which good-quality spectral data are
available. The presented method is however also applicable in the case of the
sources older than this, and/or the ones for which the only available
low-resolution radio data do not allow for detailed spectral ageing studies.
Our analysis indicates that the main factor precluding precise age
determination for FR II type radio galaxies regards the poorly known shape of
the initial electron energy distribution injected by the jet terminal shocks to
the expanding lobes/cocoons. We briefly consider this issue, and conclude that
the broad-band single power-law form assumed here may be accurate enough for
the presented estimates, although most likely it does not strictly correspond
to some well-defined realistic particle acceleration process. Instead, it
should be considered as a simplest model approximation of the initial electron
continuum, averaged over a very broad energy range and over the age of the
source, with the effective spectral index which may be different for different
sources.
