Hi TS. I am 40 right now, and I started TRT when I was 39, about 10 months ago. The onset of positive effects was gradual, it took about 4 months or so before I started seeing a significant difference.
They started me on a fairly low dose, 120mg of test cyp weekly. This dose was increased about every 6 weeks after new lab tests. For the past 4 months I've been steady at a dose of 240mg weekly. My current levels are steady with testosterone at about 760 and free test at about 24.
The only negative side effect I've suffered at this point is that my red blood cell count got very high, to the point where now I have to donate blood about every 2 months to keep my hematocrit levels in a safe range. They told me that my testicles might shrink, but if they have it is such a small difference that neither me or my gf have noticed.
As far as the positive effects, there have been many. I have put on several pounds of muscle mass, and lowered my bodyfat at the same time. The gains aren't crazy good or anything, but compared to other friends my age they are better. My mood is much better. I sleep better. I can eat more foods I enjoy without worrying about my weight. (I probably don't eat as well as I should, but I want to enjoy my life so whatever.) My recovery from both injury and workouts has improved noticeably. My strength has improved decently, and my muscle density is also noticeably better.
My sexual function is much better, similar to being in my early 20s. I can easily perform 3-4 times a day, all with a very solid erection. I look better, I feel better, and find myself being more assertive and enjoying life more. I have tried to talk to my doctor about increasing the dosage, but he won't move me up past the levels that I'm on now. Honestly, I don't really need it, that's just me being greedy. I definitely am happy with my decision.
You should be aware that some people have more side effects, but your doctor should be able to adjust your regimen or offer other medications to counter them. Best of luck with whatever you decide to do. If you want to mssg me with any questions that you feel are too personal for public consumption on this board, feel free to do so.