If you feel that your boss’ age is compromising his ability to do his job, bring the subject up gently. If you’re too direct he may become defensive. Also consider that he may not have anything else in his life to replace his job, and that letting go may be difficult.
One option is to approach the subject indirectly. Is there anything he’s always wanted to do but hasn’t got around to yet? Any hobbies or interests? Also, emphasizing parts of his job he finds stressful may remind him that if he retired he wouldn’t have to deal with them any more.