At least he'll look adorkable! :) I have no problem with the sideburns. And after playing another detective for three years, Jamie may embrace anything that will differentiate Caolan from Matt in appearance.
I didn't watch Smallville beyond the first season, but did watch Kristin when she guest-starred on Chuck and she didn't come off as young to me. I think the material and her performance will help age her. Or, ya know, she cast a spell to shave off a few years. ;)