Hi Tres Bien Merci,
They only check your face and hands, but will not roll up the sleeve, just want to see the front & back of your hands that are not covered by the uniform.
I would say they might prefer younger girls, else they wouldnt conduct talks in different polytechnics. But it's hard to say, cos I am 26 & still trying. My friend who is 28 got in this year too.
As for skin wise, I reckon as long as you look healthy, then no issue. I've met people who are tanned and skinny but got selected too.
One must be optimistic!
If you really want the job, then you should fight for it! Good luck