Context : Determination of ages of powerful radio sources is crucial in understanding galaxy evolution , the activity cycle of galactic nuclei and their influence on the surrounding intergalactic medium .
So far , several different methods for age estimates of classical double radio galaxies have been proposed and widely used in the literature , although each of them faces some difficulties due to observational limitations and/or freedom in choosing the underlying model assumptions .

Aims : We propose a new approach in determining ages of FR II type radio sources , on one hand exploiting a dynamical model developed for these objects by Kaiser et al .
( 1997 ) , and on the other hand using multifrequency radio observations not necessarily restricted to the high-resolution ones .

Methods : In particular , we apply the assumed dynamical model 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 ( like the jets ’ kinetic power ) of the analyzed sources .

Results : 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 .
Interestingly , the estimated ages always correspond to the realistic values of the jets ’ advance velocity \sim 0.01 c - 0.1 c

Conclusions : 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 , however within the relatively narrow range p = 2.0 - 2.4 suggested by our modeling .