Mental permissions have a distinct impact on the vocal range you produce. The voice is a peculiar thing - our sense of identity is tied up in it. How do we expand our sense of what is possible? Releasing tension is a process of vocal training, often, but integration is a function of working with your mind.
Let's think about this: as a child, you could flatten a restaurant with one big howl. It took very little effort. As an adult, it takes BIG effort to project your voice so freely, and yet your muscles are technically stronger. What gets in the way of the voice being powerful and effortless as we age?