The authority, exactly
The Department decides both the employer's nomination and the visa. A second stream, Temporary Residence Transition, exists for people who already hold a subclass 457 or 482 visa: instead of the Direct Entry conditions below, it requires having held that visa while working for the sponsor full time for usually at least 2 years, plus a nomination the Department approved in the 6 months before the application — the Department's page for that stream does not state a separate age limit. Where a skills assessment is required, it is issued by the occupation's own assessing authority, not by the Department.