Frustrating (but simple) math problem

I found this on a "puzzle-a-day" calendar Christmas gift. The math required to solve it is 8th grade algebra. But it is agonizingly frustrating...

"Ellie is 33 years old. This is three times as old as Sophie was when Ellie was as old as Sophie is now."
How old is Sophie now?

Try it. Get a pencil/pen and paper (you'll need it). You'll think a sentence or some data is missing. It isn't - everything necessary to solve the question is in those two sentences.

Answer in the "comments" section. Don't peek until you sweat it out for 10 minutes.