I've ranted above about how wrong Leslie Howard was for the role of Ashley, who should indeed have been in his 20s at the start of the film (in the book, he'd been back for two years from a 3-year grand tour of Europe, and I think he had been to university as well). Mitchell described Scarlett's infatuation with him this way in Chapter 2 of the book:
And so on... I've always pictured Ashley as resembling Cary Elwes in The Princess Bride, even before I knew who Cary Elwes was.