Persistent coronal imbalance (PCI) is a recognized complication following posterior spinal fusion (PSF) for Lenke 5 and 6 idiopathic scoliosis. Although adult idiopathic scoliosis (AdIS) typically exhibits greater lumbosacral stiffness than adolescent idiopathic scoliosis (AIS), direct comparative data are limited. This study aimed to compare the incidence of PCI and SRS-22r outcomes between AIS and AdIS patients. We hypothesized that AdIS patients would demonstrate higher rates of PCI and inferior clinical outcomes due to increased lumbosacral curve stiffness. We retrospectively reviewed 201 patients with Lenke Type 5 or 6 idiopathic scoliosis who underwent PSF from January 2016 to October 2023, with a minimum of two years follow-up. Propensity score matching yielded 38 AIS–AdIS pairs. PCI incidence, radiographic parameters, and SRS-22r scores were analysed. Immediate postoperative imbalance occurred in 21.1% of AIS and 31.6% of AdIS patients (p = 0.297), whereas PCI at ≥24 months was observed in 10.5% vs. 18.4% (p = 0.328). Coronal balance (–6.7 ± 11.2 mm vs. –9.6 ± 11.3 mm, p = 0.268), C1–CSVL, UIV tilt, T1 tilt, and clavicle tilt were comparable between groups (p > 0.05). AdIS patients had significantly greater residual LIV tilt (–11.8° ± 5.6° vs. –8.5° ± 6.3°, p = 0.019) and LIV–S1 angle (–6.6° ± 5.9° vs. –3.8° ± 6.0°, p = 0.042) than AIS patients. Postoperative SRS-22r scores were generally comparable between balanced and imbalanced patients. Within the AdIS group, balanced patients achieved higher MCID rates for pain (26.7% vs. 0.0%, p = 0.003) and overall improvement (23.3% vs. 0.0%, p = 0.006), whereas self-image MCID attainment correlated with greater satisfaction. In AIS patients, satisfaction appeared independent of coronal balance or MCID achievement. AIS and AdIS patients achieve comparable coronal balance post-PSF, likely aided by intraoperative cross-bar techniques. Greater residual LIV tilt and LIV–S1 angles in AdIS patients reflect increased lumbosacral curve stiffness, which is a critical factor influencing intraoperative correction strategy.