The assurance comes after a number of countries decided not to give the jab to those aged over 65 due to doubts about its effectiveness in that age group.
South African officials suspended the rollout in their country after a small trial suggested it may offer less protection against the variant that originated there.
Dr Alejandro Cravioto, chairman of the WHO's Strategic Advisory Group of Experts on Immunisation, said: "In the case of the data coming from clinical trials, we have seen that there was a small participation of people over 65 years of age.
"However, the results of the efficacy estimate for persons up to 65 and older had a wide confidence interval. And therefore we feel that the response of this group cannot be any different to groups that are of a younger age.
"Since we have identified 'people over 65' was one of our priority groups in the prioritisation roadmap... looking at the safety and immunogenicity data... we recommend for the vaccine to be used in people 18 years and above, without an upper age limit.