Just to elaborate a little on what @Jeff_Schneider has explained – the user object in Asana’s data model can never be premium in itself. It can only be associated with domains (Workspaces or Organizations) that are premium. A user has access to premium features only within those premium domains.
So what you’d want to be testing by trying to access a premium feature (like admins, custom fields, or start date) is whether or not the domain associated with a user is premium rather than if the user is premium. Or, if you would just like to know if the user is a member of any premium domains, you’d want to loop through their workspaces field checking each domain.
Hope that helps!