^She's not that much older. Christine Vale was born in 2343 according to Memory Beta (though I'm not sure of the source), which makes her 38 as of Over a Torrent Sea
. That's only 5 years younger than Amanda Tapping is now.
Still, I agree, I don't quite see it. Though I don't agree with the popular suggestion of Katee Sackhoff either. My mental image of Vale is sort of a composite, not based on any one person in particular.