How old is old? ... between 200 and 500 years old for a ponderosa pine. At around 120-150 years, the black bark changes to a yellowish-red color and forms flat plates. When the cracks on these plates warm, that's when you get the vanilla scent.
Another interesting tidbit: Snags stand 2-50 years before succumbing to wind, fire, or illegal chain-saws (except Chumley's).
It is important for large trees to die in place and eventually fall to the ground for many reasons but I'm not going to type them all out here as I think most of us know.