Context :

Aims : Radio galaxies with a projected linear size \gtrsim 1 Mpc are classified as giant radio sources .
According to the current interpretation these are old sources which have evolved in a low-density ambient medium .
Since radiative losses are negligible at low frequency , extending spectral ageing studies in this frequency range will allow to determine the zero-age electron spectrum injected and then to improve the estimate of the synchrotron age of the source .

Methods : We present Very Large Array images at 74 MHz and 327 MHz of two giant radio sources : 3C35 and 3C223 .
We performed a spectral study using 74 , 327 , 608 and 1400 GHz images .
The spectral shape is estimated in different positions along the source .

Results : The radio spectrum follows a power-law in the hot-spots , while in the inner region of the lobe the shape of the spectrum shows a curvature at high frequencies .
This steepening is in agreement with synchrotron aging of the emitting relativistic electrons .
In order to estimate the synchrotron age of the sources , the spectra have been fitted with a synchrotron model of emission .
Using the models , we find that 3C35 is an old source of 143 \pm 20 Myr , while 3C223 is a younger source of 72 \pm 4 Myr .

Conclusions :