Because author William Faulkner uses so many time shifts in his short story, "A Rose for Emily," Emily's age is not specifically set in any of the five sections. The story is told in a non-linear manner, and the stages of Emily's life follow suit.
CHILDHOOD. There is no information regarding Emily's early childhood. We know that the Grierson home was built in the "lightsome style of the seventies (1870s)" (Part I). However, Emily was probably already well into her teens or older at that point.
ADOLESCENCE. Again, there is nothing specifically said about Miss Emily's teen years. The picture of her father and herself are but a hint to her youth.
None of the young men were quite good enough for Miss Emily and such. We had long thought of them as a tableau, Miss Emily a slender figure in white in the background, her father a spraddled silhouette in the foreground, his back to her and clutching a horsewhip, the two of them framed by the back-flung front door.
However, Miss Emily could have been into her 20s at this point (Part II). We can assume from the picture that Mr. Grierson dominated Emily quite thoroughly and discouraged most of her suitors, which explains her spinsterhood as she reached her 30s.
MID-LIFE. The story focuses mostly on this stage of Emily's life, beginning with her father's death and her reluctance to give up his body (Part II); this is followed by the era of Homer Barron and their short courtship (Parts III and IV); next comes the purchase of the rat poison (end of Part III); then, the mystery of "the smell" (beginning of Part III); and, finally, Homer's disappearance/break-up (Part IV).
POST MID-LIFE. References to her later years are scattered through several chapters, including her china-painting lessons (Part IV); the remittance of her taxes (Parts I and IV); her death (Parts I and V); and aftermath (Part V).