I have met quite a few guys on testosterone replacement after testing very low for testosterone and thyroid.
Andropause is the new term for the midlife downturn in a man's energy/libido/mental acuity.
Testosterone is very important in preventing osteoarthritis and heart disease. It also prevents skin from thinning as it ages...you know how older folks develop tears in their skin easily from simple things like wearing glasses?
I am on bio-identical testosterone cream, as, it is also very important for women, although in smaller doses than what men need. I found a huge improvement in mental acuity, physical stamina, libido (had to cut back a bit

) and muscle definition, although not like body builders or anything...I take estrogen/progesterone cream and without testosterone I lose muscle tone, despite working out, as those female hormones are about packing on the fat.
Bio-identical creams are reliable and fairly cheap. They are made in compounding pharmacies.
The only drawback from testosterone is acne, hair loss and whisker growth (and Dad never taught me to shave) which can be alleviated by lowering the dose.
The most amazing thing I felt after only two weeks of taking it was a renewed sense of 'self' and purpose.