It’s tempting to think that bringing testosterone back to “healthy youthful levels” will mean that a return of overall youth and fitness will follow. But it probably doesn’t work that way UNLESS the person in question actually IS youthful with a medical problem that has resulted in abnormally low testosterone. For the older man (say 50+), youthful levels may be neither normal nor healthy in a body that is no longer young in every other respect. The point here is, tread with caution. There could be serious, unforeseen downsides–just as there have been with HRT for women.
Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.