Como dizer "verde (em oposição a maduro)" em inglês

Você pode fazer uso de "green", que não é apenas a cor, mas também o estágio em que está o fruto: verde. Ou seja, "unripe".

Oxford: green: not yet ready to eat.
Merriam-webster: green: not ripe yet.
Wordreference: green: not fully developed or matured; unripe.

''If the fruit is green, it means it is not ripe and you should leave it to ripen at room temperature.'' [Google]
“When it's fully ripe, it's very, very good,” Rymph said, warning, “Never eat it when it's green. That's bad.'' [Google]

Exemplo traduzido:

This peach is still green. [Dictionary.infoplease.com]
Este pêssego ainda está verde.

Bons estudos. Compartilhe.

Thank you guys, you're really awesome!

See you around!

Hello,

Sendo bem específico, posso dizer de uma pessoa imatura e/ou sem experiência:

■ He's green.
■ He's unripe.

Thanks in advance.

Hello,

Para se referir a uma pessoa você pode usar a palavra "immature" (imaturo) ou "inexperienced" (inexperiente), por exemplo.

Se você disser "he's green" o que me vem na mente é "he is green...with envy"

All the best!

When expressing that, referring to people, it could use a range of words, adjectives, verbs and related expressions.

Often they use something like "not a grown-up", crybaby, pampered, spoiled brat/child, fool, infantilized adult children and all that jazz...
Or, for instance something a la "he is not quite a grown-up yet.../isn´t a real man/acts like a boy" (or not like a emotional grown-up, if you wish).

And the Daily Mail online said that about Dad Brad and ex-wife Jolie:
...one might hope these two demi-adults would try harder to be good parents and keep their complicated family unit intact. Apparently not.
http://www.dailymail.co.uk/femail/artic ... ew-up.html

Well, that´s just an example, one knows that the UK tabloids can be harsh sometimes, or at least give someone a hard time!

Ah, and there is the "wet behind the ears" Generally, said about a young and inexperienced person at the job.

This term alludes to the fact that the last place to dry in a newborn colt or calf is the indentation behind its ears. [Early 1900s]
Source: The Free Dictionary.