A life extensionist is a term slightly distinct from transhumanist in that it indicates an individual primarily interested in slowing aging and extending life spans through relatively conventional means. This means to the implicit exclusion of other more radical technologies involved in life extension such as cyborgisation, mind uploading, and similar technological areas.
Life extensionists will typically not be singularitarians. If they are, they would typically identify instead as transhumanists.
A scientist working in the field of life extension may be a biogerontologist if they are studying the biological process of aging.