not a dumb question at all, since the scenario you brought up is a permutation that cannot be disregarded.
just as some types of rootstoc(for other species)have dwarfing tendencies and relatively shortened lifespans on the scions grafted to them, hj could be just that on itself. HJ is actually dwarfish compared to other jujus, even when grafted on other rootstock which supports rapid growth for other cultivars.
in the tropics, airlayered citrus is generally not capable of rapid growth, and will assume a bushy habit which will take years to form thick-enough branches to be called trunks. When grafted to seedlings of same species(that are self-fertile, say, calamondin), trunk formation is faster, but a calamondin seedling that is not top-worked tend to form a main trunk, assuming a relatively tree-like habit with a thick and long trunk prior to forming a canopy, and the thorns will be formidable(not generally seen on airlayers/scionwood grafts)
could almost surmise that the age of the roots is the true age of any tree, which could be ‘diluted’ by grafting over with budwood that has matured(if no watersprouts are permitted to grow). Also think that it may go the other way around – with rootstoc diluting the age of the budwood(and also surmise that watersprouts are always of the same age as the rootstoc and not influenced by budwood grafted onto it)