Comment by JdeBP
2 days ago
It does not have to be. The English language has a process where phrases become hyphenated compounds which then become single words. It's permissible to be partway along that path, and for people to disagree where something is on that path.
Pick any point in the past few centuries, and there's going to be something, possibly nowadays always a single word, but not necessarily so even now, that was in a state of flux at the time. The same goes for today.
“Old growth forest” is incorrect in any formulation of English grammar that I'm familiar with. It's not about fixed phrases. It's about using an adjective+noun pair (in this case, old+growth) as an adjective modifying another noun (in this case, forest). This is a general rule that applies across the board, not an isolated phrase example.
The poster was correct in asking what a “growth forest” is, because without the hyphen, the phrase parses as an adjective (old) modifying a compound noun.
And "nowadays" is an example of that process https://books.google.com/ngrams/graph?content=nowadays,%20no...