Yes a decline in testosterone is normal as a man ages, but it should be a slow steady decline. It seems these days, more and more men as young as their late 20's are experiencing acute declines in their levels of testosterone due to a number of factors. Poor diet, obesity, bombardment of environmental estrogens, excessive hormone content in meat products, certain prescription drug usage and lack of excercise all contribute to lower testosterone levels.
What are the symptoms and how do you know if your testosterone is low? Here is a list of symptoms that are associated with low testosterone levels. If you have a few of these it is worth having your bloodwork done to determine if your levels are low. Weight/fat gain particularly in the belly and chest area, little or no sex drive, erectile dysfunction, sore joints, constant fatigue, lack of interest in pretty much everything, depression, on occasion-tender nipples due to elevated estrogen, mental fatigue-loss of mental clarity, and loss of physical strength. These are all typical signs of low testosterone. At 37 a man should not have most of the problems listed, can happen, but generally shouldn't. If a man has these symptoms he is pretty much going to feel like crap. That's where HRT comes in. The doctor will try to get your levels back to a healthy range. Typically they shoot for a level consistent with a 21 year old male which is anywhere from 600-800ng/dl. The extremely low range is 241ng/dl or below. The "normal" range is 241 -100ng/dl, but you have to consider normal for one man might be 400ng/dl where as for you it may be 700ng/dl. It is better to be the higher side of normal than the lower. During and after treatment you'll notice a lot of the symptoms start going away and within about 3-4 weeks you start to feel really damn good again. Sometimes you can get off HRT and if you keep your diet clean, exercise and minimize estrogen exposure you can come off the therapy and maintain your testosterone levels give or take a few points flucuation. I think the majority of men, however stay on therapy for the duration of their life. usually these are the older men, but can happen to guys in their 30's as well.
Hope this answered some of your questions