Former United goalkeeper Edwin van der Sar was 40 when he retired in 2011 and Ferguson has pointed to other examples of players enjoying long careers.
He said: "Peter Schmeichel was 27 when he joined us. Everyone was talking about a young goalkeeper joining us but he was 27, and he gave us eight great years.
"Goalkeepers can last longer. If they look after themselves even better, but as long as they keep their agility and their speed they can play to a good age."