  We present Giant Metrewave Radio Telescope (GMRT) 240 MHz observations of the
nearby luminous FR I radio source 3C 270, in the group-central elliptical NGC
4261. Combining these data with reprocessed Very Large Array (VLA) 1.55 and 4.8
GHz observations, we produce spectral index maps that reveal a constant
spectral index along the jets and a gradual steepening from the ends of the
jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted
to the integrated spectrum of the source gives an asymptotic low-frequency
index of $\alpha_{inj}=0.53_{-0.02}^{+0.01}$, while JP models fitted to the
observed spectral index trend along the lobes allow us to estimate radiative
ages of $\sim29$ Myr and $\sim37$ Myr for the west and east lobes respectively.
Our age estimates are a factor of two lower than the 75-Myr upper limit derived
from X-ray data (O'Sullivan et al. 2011). We find unlikely the scenario of an
early supersonic phase in which the lobe expanded into the ISM at approximately
Mach 6 (3500 km s$^{-1}$), and suggest that either the source underwent
multiple AGN outbursts with possible large changes in jet power, or possibly
that the source age that we find is due to a backflow that transports young
electrons from the jet tips through the lobes toward the nucleus relatively
quickly. We calculate that in the lobes the energy ratio of non-radiating to
radiating particles is $\sim4-24$ indicating significant gas entrainment. If
the lobes are in pressure balance with their surroundings, the total energy
required to heat the entrained material is $10^{58}$ erg, $\sim$40% of the
total enthalpy of the lobes.
