Working length determination in elderly patients may be more difficult because of which of the following?
Radiographs,  tactile sensation,  the presence of moisture on paper points, and knowledge of root morphology have been used to determine the length of root canal systems. Currently, the electronic apex locator is considered an accurate tool for determining working length. One study reported that the use of electronic apex locators in a dental student clinic resulted in a higher quality of obturation length control and an overall reduction in the number of radiographs taken. However, these devices must not be considered flawless, because several variables are known to affect their accuracy. For example,  immature roots can present problems. Once roots mature (i.e., form a narrow apical foramen) and instruments are able to contact the canal walls, an electronic apex locator’s accuracy greatly improves.  Some investigators have found no statistical difference between roots with vital and necrotic tissue. Because apical root resorption is prevalent in necrotic cases with longstanding apical lesions, it may be concluded that apical resorption does not have a significant effect on the accuracy of electronic apex locators.
Some clinicians have advocated the use of the electronically determined working length in lieu of working length estimations using the placement of a file in the canal and a radiograph. However, combined use of both of these techniques has been shown to result in greater accuracy. Furthermore, radiographs may add essential anatomic information that could be missed if electronic apex locators are used exclusively.
Working length determination in elderly patients may be more difficult because of increased cementum deposition, modifying the apical anatomy.
Ref: Cohen pathways of Pulp 11th ed page no 237
Increased bone density, making radiographs harder to interpret
Increased cementum deposition, modifying the apical anatomy
Differences in tissue electrical resistance, making apex locators less accurate
Patients are unable to sit still for radiographs