He could have been in a non-combat role, such as quartermaster. It was not uncommon for older individuals to serve in support roles, such as quartermasters. The role of a quartermaster was crucial, as they were responsible for supplying troops with everything they needed, from food and clothing to equipment and transportation.
I have encountered a few instants where someone of that age joined, and was discharged for being physically unfit for service. So it would be possible for someone to join, as long as they could perform the duties.