The only relevance bodyweight would have on the calculation is after determining the person’s bodyfat. That is, one can then use the estimated percentage for determing lean vs. fat mass. I know this sounds elementary for most, but just so we’re on the same page.
As far as age, studies show that bodyfat increases with age. Now, the reason that age would thrown into the body density equations (i.e. Jackson-Pollock) is because the increase in age-related bodyfat is not subcutaneous but rather visceral. This is more of an intuition than fact, though.