The term is vague. Since a great many spruce trees (etc) live to be several hundred years old (4-500), the wood is typically older than that. In my experience, it means that the wood was *cut* 100 years ago (or whatever) and consequently has been cut and drying since then. In the main, time from cut is important, because that’s when the wood starts to dry, but it’s not a guarantee – wood must be treated and stored properly to age well without cracking or developing internal shakes.