Suppose you cut a five year old's hair, save it for 10 years, and then use it for a Polyjuice transformation. What age do you turn into: The toddler or the teenager?
TWO POSSIBILITIES EXIST
You would appear according to the age of the sample when taken
As evidenced in this question
DNA degradation due to aging. When discussing aging in biology, we use the technical term senescence. Of interest here is the shortening of telomeres due to mistakes made in your cells as they continue to divide (copying the DNA to each new daughter cell).
Since PolyJuice potion requires genetic material in order to work, it stands to reason that you would gain the appearance based on the state of the DNA/RNA avalaible in the sample. If the genetic material is from an older person, the material would be more degraded (aged) and the resultant appearance would reflect that.
The Potion could extrapolate an approximate age of the target based on the state of degradation in the genetic materials, developing an image based on the genetic material that reflects this degradation.
Mentioned is Mad-eye Moody being kept alive by Barty Crouch, who was impersonating him. Crouch needed him alive so he could harvest new genetic material as his supplies on hand ran out. Likely maintaining a stock of viable organic material is difficult for him to have, so he keeps him around to periodically collect more hair/skin/fingernails/etc.
PolyJuice is Voodoo related
There may be a magical component as well, as the genetic material, while reproducing age, would NOT reproduce non-genetic traits such as hairstyle, scars, markings, tattoos, etc as is shown in both books and movies.
To that end, the genetic material merely creates a link between it and the person targeted, as a Voodoo doll would do. In this case, the potion would then mimic the exact current state of the targeted person, including age and other alterations.
Barty Crouch keeping Mad-eye Moody alive is (admittedly weak) evidence for the impossibility to use old saved body parts.
Average person has about 100 000 hair on their head, not to mention the amount on the whole body. Hermione, leading expert in everything, assumed that a single hair is enough for the potion to work, and that assumption is not contradicted later in the story.
That means, that even if Moody lost half of the hair on his head and every day shaves clean the remainder of his body Barty Crouch would still have enough hair for 3 years (assuming single hair is enough for half an hour and that he keeps it up 24/7). Not to mention any other harvestable parts like layers of skin, saliva etc.
Keeping Moody alive was a high risk game that would not be played without a very good reason, and keeping the ability to talk to him looses it's allure as the time goes by.
edit: Well apparently there is nothing in the hair to determine age. So it would have to be magic. And since its magic and the magic knows if a person is dead or not(that is why Moody was kept alive) then you would polyjuice into the age the person currently is(or not work at all if dead) Thanks for the helpful comments!