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?

  • 6
    The fact that Barty Crouch, Jr., needed to keep Mad-eye Moody alive, implies that the hair must be newly cut for the potion to work. So I don't think the question arises. Sep 10, 2015 at 21:35
  • Ew. Saving hair for 10 years.
    – Anoplexian
    Apr 1, 2016 at 15:57

3 Answers 3



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.

  • The q/a you refer to offers no evidence that one could look at dna and identify the age of a person. At best, you could identify that they are an adult or a child, but not that the dna samples were taken 10 years apart.
    – phantom42
    Sep 10, 2015 at 15:18
  • True, but one could extrapolate an approximate age of the target based on the state of degradation in the genetic materials. Likewise, developing an image based on the genetic material would reflect all that degradation. Sep 10, 2015 at 15:22
  • 4
    Hair strands do not have DNA (roots will but not the hair itself)
    – user001
    Sep 10, 2015 at 16:23
  • Actually, you can tell the difference in years between two DNA samples if you apply magic to them.
    – Misha R
    Sep 11, 2015 at 5:06
  • 3
    "There may be a magical component as well". In Harry Potter? Damn the thought! Sep 11, 2015 at 7:42

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.

  • So will you be young or old? That is the real question...
    – Skooba
    Jul 10, 2017 at 20:37
  • @Skooba the point of my answer is that you will be neither as we have evidence suggesting, that such a potion would not work.
    – Deltharis
    Jul 10, 2017 at 21:13

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!

  • 2
    As discussed here, there is no evidence that the effects of polyjuice potion is anything more than cosmetic. The user may look like a 5 year old, but they would still be their real age and continue to age.
    – phantom42
    Sep 10, 2015 at 13:36
  • 1
    "Scientific" point of view?
    – Chenmunka
    Sep 10, 2015 at 13:45
  • Just that a persons DNA(or whatever that marks age) would not change in the older hair. So if used in Polyjuice it would be the 5 year old. You could argue(if you tried hard enough) that a persons hair is connected magically to their current body.
    – Tdmut
    Sep 10, 2015 at 13:59
  • 1
    Sorry, DNA DOES change as you age. Evidenced here Sep 10, 2015 at 15:05
  • 1
    Not that i know anything about it but from what they are saying i gather that you cant tell a persons age by their DNA. Not that it doesnt change while you age.
    – Tdmut
    Sep 10, 2015 at 15:08

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