Jackman is entirely too old to play Ken, even assuming he'd be interested. Ken and the Princess are meant to be the juvenile/ingenue couple playing against the older couple, Sally and Cosmo. That's the whole point of the counterpoint number "You're Just In Love." Ken's having all these feelings that he doesn't understand, but Sally does recognize and understand, because she's older and more experienced, and proceeds to explain it to him. If Sally and Ken are contemporaries, the song loses its point.