The former Arsenal captain has proved the difference for United in this season's title race.
Ferguson, asked if the Dutchman's age was a barrier to signing him, said: "That was certainly an issue. It's always an issue for me, in particular.
"In the past when we have bought older players it's not always worked. But with Van Persie I was confident it would work. He's a mature player.
"Will he go down as one of our greatest every signings? I hope so. I think he's got the chance to kick on now. And I think his personality has got bigger and stronger as the season has gone on."