  We use the new QUIJOTE-MFI wide survey (11, 13, 17 and 19 GHz) to produce
spectral energy distributions (SEDs), on an angular scale of 1 deg, of the
supernova remnants (SNRs) CTB 80, Cygnus Loop, HB 21, CTA 1, Tycho and HB 9. We
provide new measurements of the polarized synchrotron radiation in the
microwave range. For each SNR, the intensity and polarization SEDs are obtained
and modelled by combining QUIJOTE-MFI maps with ancillary data. In intensity,
we confirm the curved power law spectra of CTB 80 and HB 21 with a break
frequency $\nu_{\rm b}$ at 2.0$^{+1.2}_{-0.5}$ GHz and 5.0$^{+1.2}_{-1.0}$ GHz
respectively; and spectral indices respectively below and above the spectral
break of $-0.34\pm0.04$ and $-0.86\pm0.5$ for CTB 80, and $-0.24\pm0.07$ and
$-0.60\pm0.05$ for HB 21. In addition, we provide upper limits on the Anomalous
Microwave Emission (AME), suggesting that the AME contribution is negligible
towards these remnants. From a simultaneous intensity and polarization fit, we
recover synchrotron spectral indices as flat as $-0.24$, and the whole sample
has a mean and scatter of $-0.44\pm0.12$. The polarization fractions have a
mean and scatter of $6.1\pm1.9$\%. When combining our results with the
measurements from other QUIJOTE studies of SNRs, we find that radio spectral
indices are flatter for mature SNRs, and particularly flatter for CTB 80
($-0.24^{+0.07}_{-0.06}$) and HB 21 ($-0.34^{+0.04}_{-0.03}$). In addition, the
evolution of the spectral indices against the SNRs age is modelled with a
power-law function, providing an exponent $-0.07\pm0.03$ and amplitude
$-0.49\pm0.02$ (normalised at 10 kyr), which are conservative with respect to
previous studies of our Galaxy and the Large Magellanic Cloud.
