Good questions. I don't think that as we age we only lose test levels... so I don't think TRT by itself will be enough to keep the gains.
I caught a glimpse of a TV show years ago - and don't remember where - that talked about a life extension clinic. One of the "patient" was 72 and looked like a muscular 45 yo.
They used a cocktail of SMALL doses of 7-8 hormones: test, hgh, melatonin and a few others I don't remember. It still seems to make sense and sounds better than a "blast test" approach, at least for health and anti-aging.