Lifestyle changes are one a reason. We used to be more active, and be out in the sun more which generates greater Vitamin D exposure. Having an indoor job, such as an office job, would definitely result in lower T levels without enough exercise and Vitamin D.

Another thing to consider is the abundance of microplastics. They are endocrine disruptors, and are shown to hurt T levels as well. Along with modern food processing ingredients like soy letchin showing to hurt testosterone levels, it really seems like we created this environment.

On another note, we know testosterone is intrinsically involved in voice maturity and facial hair, which partially leads me to believe it's responsible for more of your appearance.