I don't think green necessarily implies someone is young, usually just inexperienced. However, you're correct, you don't hear it used with older people as often. Perhaps it's just because people who are older typically naturally have more experience. If you called a middle-aged person who was inexperienced, "green", it would make sense and I wouldn't question you.

Somewhat disagree with the other answers because this use of "green" presumably comes from the plant world, and relates to a lack of time to develop/ripen. I wouldn't use "green" this way for any age, though, as it's vague and ambiguous. I'd be more likely to use "greenhorn"

Being "green" has nothing to do with age. It's just about time/experience in the specific topic, as Alan says.

Thank you very much, Alan!

As a native english speaker I've never thought of "green" as having anything to do with age. Rather amount of time/experience in the specific topic.