Sir Alex will celebrate his 67th birthday on New Year’s eve.
Bobby Robson was sacked at Newcastle at the age of 71.
“I won’t be doing a Bobby Robson and be a manager when I am 70,” he told the Mail on Sunday.
“It is just knowing when to quit.
“Football is like a drug which is difficult to give up.
“My appetite and enthusiasm are as strong as ever. I love watching Manchester United. We have some terrific players who are only going to get better.”
Inter Milan coach Jose Mourinho has been touted as a possible replacement for Ferguson.