Age at first sexual intercourse and frailty: Mendelian randomization and population-based evidence from NHANES and UK Biobank - Kai-Xian Wang; Bao-Peng Liu; Jia-Ning Wang; Kai-Zheng Wang; Yi-Zhan He; Meng-Yao Yu; Zi-Ming Shao; Long Sun; Mei-Xia Xu (Sexual Medicine, 2026)