BackgroundOlder adults comprise 40% of surgical inpatients and are at increased risk of postoperative rehospitalization. A decade ago, 30-day rehospitalizations for Medicare patients were reported as 15%, and more than 70% was attributed to medical causes. In the interim, there have been several large-scale efforts to establish best practice for older patients through surgical quality programs and national initiatives by Medicare and the National Health Service. To understand the current state of rehospitalization in the USA, we sought to report the incidence and cause of 30-day rehospitalization across surgical types by age.Study designWe performed a retrospective study utilizing the American College of Surgeons National Surgical Quality Improvement Program (ACS NSQIP) dataset from 2015 to 2019. Our primary exposure of interest was age. Patients were categorized into four groups: 18–49, 50–64, 65–74, and 75 + years old. Reasons for rehospitalization were evaluated using NSQIP defined causes and reported International Classification of Disease (ICD)-9 and ICD-10 codes. Our primary outcome was the incidence of unplanned 30-day rehospitalization and secondary outcome the cause for rehospitalization. Variables were summarized by age group through relative (%) and absolute (n) frequencies; chi-square tests were used to compare proportions. Since rehospitalization is a time-to-event outcome in which death is a competing event, the cumulative incidence of rehospitalization at 30 days was estimated using the procedure proposed by Gray. The same strategy was used for estimating the cumulative incidence for unplanned rehospitalizations.ResultsA total of 2,798,486 patients met inclusion criteria; 198,542 had unplanned rehospitalization (overall 7.09%). Rehospitalization by age category was 6.12, 6.99, 7.50, and 9.50% for ages 18–49, 50–64, 65–74, and 75 + , respectively. Complications related to the digestive system were the single most common cause of rehospitalization across age groups. Surgical site infection was the second most common cause, with the relative frequency decreasing with age as follows: 21.74%, 19.08%, 15.09%, and 9.44% (p< .0001). Medical causes such as circulatory or respiratory complications were more common with increasing age (2.10%, 4.43%, 6.27%, 8.86% and 3.27, 4.51, 6.07, 8.11%, respectively).ConclusionWe observed a decrease in overall rehospitalization for older surgical patients compared to studies a decade ago. The oldest (≥ 75) surgical patients had the highest 30-day rehospitalization rates (9.50%). The single most common reason for rehospitalization was the same across age groups and likely attributed to surgery (ileus). However, the aggregate of medical causes of rehospitalization was more common in older patients; surgical and respiratory reasons were twice as common in this group. Rehospitalization increased by age for some surgery types, e.g., lower extremity bypass, more than others, e.g., ventral hernia repair. Future investigations should focus on interventions to reduce medical complications and further decrease postoperative rehospitalization for older surgical patients undergoing high-risk procedures.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13741-023-00313-3.