That should have been hyphenated then. https://en.wikipedia.org/wiki/Old-growth_forest
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.
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.
Though I still lament the loss of the subjunctive form. And diaeresis.
It's one step short of the German compound noun, and we make it easier to find the fragments...