It is older than the Tang, in fact it dates from the Northen Wei dinasty (386-534 AD) although the original collection it was part of is lost, and the ballad survived in another, much later, opus. I find it remarkable in many ways.
木 兰 辞
Jiji fu jiji, Mulan dang hu zhi.
Bu wen jizhu sheng, wei wen nü danxi.
Wen nü he suo si? Wen nü he suo yi?
Nü yi wu suo si, Nü yi wu suo yi.
Zuoye jian juntie, kehan da dian bin,
Jun shu shier juan, juan juan you ye ming.
Ah ye wu da er, Mulan wu zhang xiong.
Yuan wei shi an ma, cong ci ti ye zheng.
The Ballad of Mulan
Whirr-clack and again whirr-clack, Mulan weaves facing the door.
(But now) no sound from the loom is heard, only (our) daughter’s sighs.
“ Daughter, what are you thinking of? What are you brooding over?”
Nothing I’m thinking of, nothing I’m brooding over.
Yestereve I saw the army register, the Khan is levying the troops.
The register is twelve scrolls, each one bears father’s name.
Father has no first-born son, Mulan has no elder brother.
I wish to buy horse and saddle, soldiering in father’s stead.
Mulan is an enormously popular character in Chinese folklore, it all started from this ballad, composed about 1500 years ago. From here came a novel written during the Ming dinasty, more poems, theatrical plays, TV series, live-action movies and cartoons, including the Disney one that gave Mulan popularity in the West (the downside of it is that many schoolchildren over here think Disney invented Mulan).
I liked the Disney movie for many reasons, and honestly I don't mind the chronological mish-mash overmuch, after all in China beloved stories are told and retold and undergo many transformations, so long as the 'layering' is evident and the different strata are there if one digs, I have no problems with the process.
But I like the original better, and some of the reasons are right there in the first stanza. For starters, Mulan is no freak: no girl out of place trying to conform to the norms of society and failing. She enters the scene fulfilling the expected duty of an unmarried daughter, weaving for the family, but is pondering on a problem,the call to arms her family cannot answer, and comes up with her own solution.
Mulan doesn't steal out of the house during the night. She has a plan and executes it with the full knowledge and consent of her parents. The Northern Wei was a troubled time, historians say that it's very likely that women (specifically in Northern China where the ballad originated) received weapon training as a matter of course, in Wei statuary there are images of female warriors. So why is she passing herself off as a man? I think (and it's all speculation on my part, mind you) that it may depend on a technicality.
The ballad says that the army scrolls bear father's name and also Father has no first-born son, Mulan has no elder brother, my guess is that a son could fulfill his father's duty if the latter was incapacitated but there was no written rule about a daughter doing the same, so Mulan assumes a man's identity (later works say she takes her younger brother's name) to avoid a possible refusal.