immature

Occurring before the proper time; untimely, premature (especially of death).

And thou also canst best account for the causes of her immature death […].

Not fully formed or developed; not grown.

Childish in behavior; juvenile.

You're only young once, but you can be immature the rest of your life.

The man was immature for throwing a tantrum.

An immature member of a species.

There are many genera and even families of Brachypylina for which immatures are not yet known, and thus numerous examples of adult convergence and misclassification remain to be revealed: such is the case with Hypozetes.

While on a walk the next morning I found what looked like a patch of old growth habitat - perhaps somewhere the fires had missed - and to my astonishment saw a female Red-lored Whistler accompanied by an immature.