I don't think it is too early to introduce topics such as racism, sexism, sexual assault, etc.; however, because the book demands the reader to read between the lines to find the hidden meaning in each vignette, I'd say 8th grade is a little soon to ask for that kind of analytical reading. I teach Mango Street in my 9th grade honors class and some of them struggle to find meaning in what otherwise appears to be rather simple stories. Overall, it's a quick read most 8th graders could get through, but my concern is they wouldn't get much out of it.
I think it is appropriate for 8th graders. I think we do students of that age a disservice when we gloss over or hide issues of adulthood, especially as regards poverty, sexuality or growing up that many 8th grade students can relate to. We cannot dictate what they do and do not think about any more than we can dictate the rate of their physical and mental development. It's a puritanical streak in our society that is sometimes very frustrating to me.
I agree with the above post in that an 8th grader would be able to understand the basic story; however, the issue dealing with Esperanza's rape and abuse and other scenes regarding sexual violence are not appropriate for a middle school reader. These issues are best left to high school readers who are beginning to become mature enough to handle this type of content. There are plenty of young adult novels written for middle school students, so there's no reason to force this one.
The previous response's closing comments were extremely valid. In the final analysis, it is dependent on the teacher and how comfortable they feel in teaching the material and concepts to any grade of child. I do believe it is highly appropriate for 8th graders, but this is contingent on the instructor's willingness to engage students in as open of a discussion of identity, narrative, voice, and exploration as Cisneros poses to the reader. Certainly, if the instructor is not comfortable bringing this discussion into the open to students, any work, not just Cisneros', will be rather uninspiring and probably ineffective. Yet, if the instructor is able to bring out contextual elements which present themselves in the story and develop an understanding about these ideas in a larger group, the work can be quite powerful for many different age groups of students.
The language of the book is certainly simple enough for 8th graders, but we teach the book in our school district in the 10th grade because we focus on the complexity of Cisneros's themes and style. An 8th grader could comprehend the book, and I think that the ideas and topics in the collection are appropriate for that age, especially since Esperanza is observing other "women" so that she knows how to be a woman.
So, the answer to your question depends on what a teacher wants students to focus on in the book.
I feel that a higher level 8th grader could easily comprehend the book, although I wouldn't recommend reading it because the vignettes' meaning go far beyond its words. There are hidden meanings and contexts that the story is put in. A vignette that involves little girls putting on high heels may also be there to serve a different purpose when analyzing it, because that vignette also goes into depth (although not written/shown) about desire for maturity and sexuality. Sally herself represents a mature, sexually-compelling girl. There are also many hidden messages in the vignettes involving sexuality and promiscuity ('Red Clowns', etc) and unless you want to fully go into depth with that, I wouldn't suggest 8th graders reading it. Because when looking at Sandra Cisneros's style of writing in this, looking into all aspects of her writing is vital. Other than that, it is perfectly fine.
I think the book is appropriate due to the characters and issues discussed. Not only does it have a female protagonist but she is also a minority-based character. She deals with many of the issues that an eighth grade student may have in their own life i.e. family relationships, sibling roles, and adolescent development. The briefness of each vignette allows for ease of reading (attention span).