This leads me to wonder... could you actually like polyjuice yourself or others into a younger version of self or others if you had some hair from the younger days?
My feeling would be, no. Rowling states that race and sex can be changed by polyjuice, she does not mention age. I have a feeling the body part of the one the taker wishes to change into needs to be harvested "fresh". Otherwise it would make no sense why Crouch Jr. keeps Moody in the trunk. He could just cut of a bushel of hair and be done with him, no?