If you could get one of your younger attributes back, which would you choose?
Still got all my hair, vision is still 20/20, although I now need reading glasses sometimes, manly parts still work, so I voted for knees and joints.
Too many years of full court basketball, with the requisite sore knees and twisted ankles. Most mornings now, I pop and crack like crazy for the first 20 feet or so.
Not sure I understand the question. Would hair count? :o
Optimism/faith in humanity.
My back!
I want my back back, as it were! (Should’ve been a choice!)
You’re right darn it! And the irony here is my back is always giving me shit!
I voted metabolism, but I’m greedy; I want my entire 20 year old body back. 
My ability to digest raw vegetables.
A functional pancreas.
This.
I voted appetite/metabolism.
Once upon a time I was a competitive swimmer. Somehow I managed to get in a mile to a mile and a half of laps during practice each day and never seemed particularly hungry.
That has… ahem… changed. Now my physique is more suited do be used as a flotation device rather than a lifeguard.
I’m with Jack of Words - I voted “other”. Has anyone seen my rose-colored glasses?
Other – naiveté
Back (ruined by way too many 10+ hour MMO gaming sessions) or teeth (bad genes and a love of sweets). I’m actually stronger now than I ever was and I much prefer my BMI 21 body to my old way too thin teenage body so no problems there … yet.
In my teens, the doctor said I was in the 90th percentile of skinniness. Ha! If only he could see me now…
My voice. I used to have a fairly decent vocal range, impressive breath control, etc.
Decades of not keeping in practice and polluting my lungs have pretty much killed all that.
Other: confidence
Having just had a knee replacement that is not providing optimal results, I want my youthful knees back.
I want my ability to work on a problem for years, if necessary, till I solve it. Just cannot do that any more. After my thesis was finished with partial results, I worked on the problem–on and off–for five years before I finally cracked it wide open. Maybe the best thing I ever did. Although I’ve done easier things that have had more impact.
I want some energy.