If you test out as deficient in testosterone, or if you have symptoms of low testosterone, you likely want to do something about it. There are definitely over-the-counter supplements designed for this very purpose. ( Alpha Male® and Tribex® are the most potent.) And while effective, they're best used by healthy younger men who want a boost in T levels for bodybuilding purposes. They probably aren't the best choice for men who are clinically low and who've made the choice to undergo what's usually a lifetime commitment to testosterone replacement therapy, or TRT.
The good news: the technology for pellets has greatly improved and most of the side effects listed in #1 have been greatly reduced with the new Testopel pellets. For example, one recent study showed extrusion and infection rates of only %, an admirable improvement to say the least.  Although the study was sponsored by the manufacturer, I think everyone agrees that these pellets are a significant advance over the prior, being smaller and with more tight quality control for contaminants leading to infection.
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.