A mature voice is all about projecting an air of grown-up confidence, a "been there, done that appeal." When you a read that will have your audience trusting the weight of experience, look no further than this amazing talent pool..
it's all about experience with this voice. A mature voice is all about a fully-developed attitude that has no time for childish games. The moms, the dads, the character who is slightly older than the rest of the group and has seen their fair share of action, or that voice in commercials that carries an air of inherent trustworthiness. A mature voice has the commanding quality of someone who has evolved past the limitations of their younger self, and is ready to share their wisdom with others.
There are characters in video games, film, and animation that sometimes could use a little talking to by a character with a mature voice. Whether it's the scolding parent, or the experienced warrior who's ready to make the full weight of their experience felt, tropes abound. Or maybe you're thinking about casting a hero or heroine with a mature voice, like Geralt of Rivia in the Witcher series. Ads and commercials often benefit from having a mature voice around too, as it conveys an immediately relatable quality to older audiences (great if you're selling a product that caters to them), and authority to younger ones.
Lived-in, precise, deliberate, but relatable. A mature voice is in complete control of itself due to the benefit of years of hindsight, practice, and experience.