There are, actually - but they are not useful in determining how short you might become.
The decrease in height comes from four different factors. Number one is wear or degradation of cartilage in ankle, knee, hip and pelvic joints, which can account for fractions of an inch. Number two is degenerative disc disease in the neck and back, which can account for, perhaps, three inches at most. Number three is scoliosis or other deformations of the spine, which can account for four inches - possibly more in the severest cases. Number four is posture, which can account for up to two inches.
Of all of these, only posture can be reversed. it might be possible to minimize the effects of the other three causes with daily care and caution.