Electron spectral index vs. age for a sample of SNRs with the radio and GeV γ-ray emission detected, from the data given by Zeng et al. (2019). The red dashed line represents a linear fit between the index and logarithm of age, with the slope k = −0.37 ± 0.05. The red area shows the 1σ uncertainty. We excluded MSH 15-56 from the sample because its radio emission is likely dominated by a pulsar. There are a few SNRs with two values of α estimated in this reference. We calculated the average α from the two values if the uncertainty is due to various estimates of the ambient density (Tycho, G166.0+4.3, S147, CTB 73B). We use only one value for W51C, RX J0852-4622, namely that which corresponds to the same spectral model as used for other SNRs.