Being physically able to do so is part of the briefing. When the person does the briefing, they are not only looking to impart the information but also evaluating the person as to whether or not they are physically capable of handling the exit and whether or not they speak an appropriate language to be able to understand the flight attendant's commands in case there is an emergency situation. They are also verifying that an infant hasn't inadvertently been placed in the exit row.
Evidence of meeting #34 for Transport, Infrastructure and Communities in the 41st Parliament, 2nd session. (The original version is on Parliament’s site, as are the minutes.) The winning word was airports.
A recording is available from Parliament.