A randomized trial gave testosterone gel to older men, followed them for 3 years and found aerobic capacity did not significantly decrease in men on TRT, but did decline in the placebo group. The abstract says:
The mean 3-year change in V˙O2peak was significantly less in testosterone-treated men than in men receiving placebo and was associated with increases in hemoglobin. The difference in V˙O2peak change between groups may indicate attenuation of its expected age-related decline; the clinical meaningfulness of the modest treatment effect remains to be determined.