Immature, a.
Etymology: [L. immaturus; pref. im- not + maturus mature, ripe. See Mature.]
1. Not mature; unripe; not arrived at perfection of full development; crude; unfinished; as, immature fruit; immature character; immature plans. "An ill-measured and immature counsel." Bacon.
2. Premature; untimely; too early; as, an immature death. [R.] Jer. Taylor.
Shades of meaning
Fernald has no entry of its own for immature; he lists it under youthful.
He lists it without comment; the entries above explain its neighbors.