India, classified as a lower-middle-income country, is witnessing a significant rise in its older adult (60+) population. Despite this demographic shift, there remains a limited understanding of the factors affecting quality of life (QOL) among older adults in India. The present study aimed to assess the QOL using the WHOQOL-BREF scale and identify the factors influencing the older adult population in the Siwan district of Bihar, India. A cross-sectional study was conducted among 476 older adults aged 60 years and older during 2021-22. The WHOQOL-BREF tool was utilized to measure the QOL, with data entry was carried out in Microsoft Excel and statistical analysis conducted using STATA v 16.0. Descriptive statistics, including frequencies, box plots, and mean ± SD, were employed to summarize overall and domain-specific QOL scores. Inferential statistics, such as t-tests, ANOVA, and multiple linear regression analysis, was used to explore the relationships between various factors and the QOL. The study found an average QOL score of 44.9 ± 8.9, with the lowest scores in the physical domain (40.6 ± 9.4) and the highest in the social relationship domain (53.1 ± 17.6). The key determinants of the QOL were found to be the age group 70–79 years [β = −6.19, 95% CI: −7.86, −4.51], aged 80 years and older [β = −10.38, 95% CI: −12.74, −8.03], single marital status [β = −2.17, 95% CI: −3.69, −0.65], being OBC category [β = 3.30, 95% CI: 1.17, 5.42], being General caste [β = 6.50, 95% CI: 3.95, 9.05], and being Muslim [β = −2.75, 95% CI: −4.59, −0.91]. The study highlighted that QOL tends to decline with increasing age, while higher caste status is linked to better QOL. Among the factors examined, age, marital status, caste, and religion were found to be the most significant determinants influencing the well-being of older adults.