Telomere shortening and the resulting cellular senescence promotes ageing in mammals. This study showed that administration (via an adeno-associated virus) of telomerase reverse transcriptase (TERT) — an enzyme that maintains the length of telomeres — to old mice decreased the incidence of age-related osteoporosis and glucose intolerance, improved neuromuscular function and partially improved memory without increasing the incidence of cancer. Furthermore, TERT therapy extended the lifespan of adult and aged mice by a median of 24% and 13%, respectively, showing the feasibility of gene therapy for ageing.