Roughly, yep, she'd appear to be right around twenty... And, actually, believe it or not, that's about the age her PB (Lily Cole) is. X-D

Given that Polly was at a much younger mental/emotional age when she arrived, that ambiguity worked for her. I've been gradually trying to phase out the youngest-looking ones and replace them as she progresses. :-)

So far as Polly's actual age, there's no real knowing, being as her home-dimension is outside of Time. She could have lived the equivalent of 20-ish years... or 200 or more.