What is a mature vocal sound?
In resonance-based styles such as those related to most classical singing, singers sometimes try to sound “more mature”. Some teachers encourage this. It is a mistake.
Maturity is something that is earned with time and action. It is something that happens, not something you can pick up in a hurry. The voice must be free, and able to do the functional things that are required of it, but making it sound like something that it is not is not good for singing.
In classical singing, one of the goals is maximum resonance. In a word, it is about loudness, but not just any kind of loudness. The sound is amplified by the vocal tract in such a way as to project in a large space, sometimes over/with many other instruments. This requires some tuning and shaping of the vocal tract, and a laryngeal suspension that is low enough to get the right shape to enhance the resonance. The resonance adjustments must not compromise the function of the voice in regards to flexibility, range, and clarity of vowels.
Consider substituting the term “optimal resonance” for “mature sound”. As you age, your resonance will probably sound more mature all by itself. But if it remains relatively youthful sounding, learn to love that. There is no need to manufacture darkness by stiffening the tongue, consciously lowering the larynx as far as it will go, or other means that may change the color, but not add to function or projection.
Yes, it is true that classical singing requires a lower larynx than other styles, but whether that lower place is the same or lower than the resting position will vary from person to person. People’s necks, throats, and oral cavities are unique. They won’t maximize resonance in exactly the same way.
If you sing in genres other than classical, similar advice applies. Don’t rush to sound old and interesting! The sound you hear from mature artists is earned by lots of experience. They probably sounded young once too.