What is likely causing the space between #29 and #30 in an elderly patient?

The presence of space between the molars, specifically between the second molar (#29) and the first molar (#30) in an elderly patient, is most likely due to mesial drifting of the premolars. As teeth age, they can experience shifts in position due to various factors like bone resorption, periodontal disease, and tooth loss. When a premolar is lost or has experienced significant periodontal breakdown, adjacent teeth can drift mesially, filling any space created by the movement of the teeth in a forward position.

In this case, the movement of the premolars toward the midline often creates a gap between the molars, leading to the described situation. The other options—impacted teeth, restorative failures, and cuspal fractures—are less likely to lead simply to a consistent space found between the molars without additional complications or scenarios. While these issues can also relate to dental problems, they typically do not result in the straightforward positional shift indicated by mesial drifting in older adults.