I see no reason to believe that Dante is the older brother.
Simply because he always won at sparring and called himself the "big brother" doesn't mean anything except that he thinks he's better than Vergil (when of course, in reality, he actually doesn't -- Hollow Dante was a facet of Vergil's personality manifesting itself as his envious side).
Until NT says otherwise, I'm still saying that Dante is the younger brother.
Vergil is set up to be the more mature, "rational" brother who leads Dante (and The Order) -- so, in that light, NT more or less implied that he's the older one.
Like I said, we'll see what NT says... if they ever get around to confirming anything about it, that is.