very good question, and one not asked enough. Gotta go now, but hope to visit later with my thoughts. Surely there are some Forestry or Botanical types who can chime in with their opinions? Scott Heatley is up there on Vancouver island surrounded by loggers/tree huggers/ old trees/ planted trees.... He'd have an interesting answer, too, i bet.
I used to cut old-growth cedar snags up near Forks, WA years ago. These guys were typically much larger than >36" DBH (sometimes 7-10ft).
I think the concept of 'old-growth' often centers around the fact that these trees are often not related to the current forest stand, which is otherwise usually a regeneration after logging, fire, or flooding, etc. Thus, old growth species are commonly older, rarer remants of the initial forest.
I see quite a few big, old, black spruce in isolated locations close to the Yukon; where seasonal fires don't get them. A buddy cut some last winter and said they were over 400 years old by rings but were only 2-3 foot across; due to the cold climate here in the Alaskan interior. I have seen birch strats & fiddles, but I never see big birch that's not rotten inside.
I wonder if black spruce would make a nice guitar body; I bet it's pretty dense, but light also.
But it still isn't clear (to me) how old the wood must be. Must it be 100, 150, 200 or more years old? What about the tree it comes from: should it be "dead" for some time, or is a 400 year old but freshly cut tree also "old growth wood"?
Old growth around these parts is forest that has not been cut commercially, which started around the late 1700's. At that time the big trees were being cut for ship masts but fairly quickly a large scale logging industry developed. Many tree farm licences are now into their fourth or fifth harvest cycle, or "growth", but it takes a unique set of conditions for a tree to live, say, 800 years and most don't. The biggest distinction between old growth wood and managed forests is that the growth rings are tighter than a forest that has been cared for, as the managed trees grow faster and are protected from stresses like fire and infestation. Whether that makes for "better" wood depends who you talk to. Old growth does make for more heat from the wood stove though!
"Old growth" is a term often used a bit ambiguously. It should mean wood cut from a mature tree, which may be 150 years old for spruce, or could arguably mean 50 years old for birch. Some people use it to describe simply wood that was harvested 30 or 50 or 100 years ago, or wood that was cut from a virgin forest, or perhaps from a second growth tree planted 100 years ago and harvested last week. It's ambiguous, the term often leaves room for interpretation.
For woods like spruce or brazilian rosewood, what people are looking for is
"virgin growth". This is from trees in a forest that has never been previously harvested. Wood from these trees will often be quite different because they grew in a different environment. They may have started under an established canopy, growing more slowly for decades in low light, having narrower growth rings, feeding from mature soil, plenty of other differing conditions from trees growing from a field that's been clear cut or burned.
I don't think you're going to find any clear answer to your question. There's no regulatory commission that defines these things, so what a wood is called probably depends most on what's selling the best.
I would contribute that the term "old growth" would vary by species, forest type and region of the world (varying with climate, soils, etc.) from which it derives. Some forests achieve maturity and approach a "climax" successional stage as early as age thirty, and others may require several hundred years with no disturbance. Old growth wood, as used by the layman, usually refers to wood that has very closely spaced growth rings and high density due to the age of the tree it was harvested from. In North American maples, this may be 100 or 150 years; in spruce grown at high altitude or in cold climates, it may be 400 years or more. Tropical species, of course, usually grow faster due to longer growing seasons, higher annual precipitation and generally more favorable conditions.
The term "old growth" seems to assume that older wood is better. Far from the truth. There are too many factors that contribute to tone woods. Weather being the main thing. Old is simply not better all of the time.:AOK