Telomere length is a core factor in aging biology and interventions to lengthen telomeres have been advocated for anti-aging. We aimed to test whether genetically determined variation in telomere length is associated with a wide range of aging-related conditions. Studying inherited genetic variants for telomere length minimizes confounding from lifestyle factors, and excludes feedback effects due to developing disease (i.e. excludes reverse causation). Subjects are naturally randomized into subgroups of these variants, which creates a natural randomized trial (a ‘Mendelian randomization’ trial) and provides more robust evidence than conventional observational studies. Associations between genetic variants strongly associated with telomere length (in TERC, TERT, NAF1, OBFC1 and RTEL1) and aging-related outcomes were tested in 250000 participants of European descent aged 60 and older at measurement. The odds ratio per standard deviation change in telomere length (approx. 4,000 base pairs) was OR=1.23 (95% CI: 1.10–1.37) for developing any cancer. Genetically longer telomeres were also associated with hypertension (OR=1.18, 95% CI: 1.09–1.29). There was suggestive evidence of increased participant mortality with genetically longer telomeres (HR=1.18, 95% CI: 1.03–1.35). However, no associations were found with healthy aging outcomes including falls, fractures, depression, frailty, sarcopenia, and hospitalization for pneumonia or delirium. We conclude that genetically increased telomere length is associated with a higher risk of cancers but no improvement in measures of healthy aging.