OK, I'm a bit late to the party, bu here's my two bits worth...

Fer starters I'd expand on what KarinaKiev briefly mentioned about head/ body proportions. Age can be subtly implied by this relationship, w/ ranges being from babies measurin' 4 heads tall (I believe) all the way to "Heroic" proportions of 8 heads, your average adult being 6- 6 1/2 heads tall if I recall my art theory correctly. Limb length also plays a factor, but is mostly only applicable to the very young or (what's the politically correct term of the day for midgets?) or fantasy races. Your full body image suggests late teens/ young adult to me by this criteria.

A 'nother point I'd make is that you have the benefit of your character being in a story (as opposed to a random single image). Use the story to describe/ explain her age/ appearance... mebbe have her get carded while goin' into a bar or sumpin' if ya don' wanna write a bunch of exposition. Just be wary of any single "sneek peek" renders ya might post of her in adult situations (if yr still bothered by the age question).

As for the hair, I kinda agree 'bout the pigtails bein' a "younger" thing in many cultures. Perhaps a single braid would be a good compromise as that can read as more "elegant" than "youthful". Or a single ponytail (unbraided) for a more casual appearance.

Luck w/ yr project :)

Yep, your art theory stuff is wrong, misremembered or whatnot.

Average = 7 1/2 heads

Ideal = 8 heads

Heroic = 9 heads... or more!

According to a couple of my reference books, anyhow.

Yup, 100% correct, just scale down some spheres to measure/use the ruler function in Poser/ or use human photo front and side references (to make sure the legs arms and torso are all correct as well.)

Everytime I look at this thread title I see

'Onion Needed on Character"