A good definition of maturity might be: choosing to ignore your first impulses for others’ sake.
Maturity is choosing to stay quiet in rehearsal when another section is rehearsing, because your impulse to talk would undermine the other section.
Maturity is choosing to practice rather than to look for Pokémon because your impulse to play Pokémon Go will undermine your future self’s success.
Maturity is choosing to keep negative opinions to yourself in public situations (like a choir festival), because your impulse to express your opinions would hurt others.
Maturity isn’t so much a function of age, though most of us find it easier to behave maturely as we get older. Maturity comes down to choice. When you choose to behave mature is entirely your decision.