De Gea has won great praise for his performance at Real Madrid last week.
Ferguson said: "Maturity helps [goalkeepers too]. There are very few goalkeepers who are the finished article at 20 years of age. There are some examples, like Gianluigi Buffon and Petr Cech. That's not always the case.
"Peter Schmeichel didn't join us until he was 27 and then gave us eight great years. Normally for goalkeepers it's the mid-20s until you get the finished article."