Comment by Timwi

“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.