What age group is this book appropriate for? Is the relationship between Guy and the older woman sexual, and if so, is it described or alluded to?

Like the other volumes of Maya Angelou's autobiographies, this one is written with an adult audience in mind. However, that isn't to say that it would be entirely inappropriate for teenage readers. This book in particular is mostly about Angelou's relationship with her son Guy as he grows older. It's written very much from the perspective of the mother trying to adapt to the change in her relationship with her children, rather than from the perspective of a son trying to navigate his relationship with his mother. There's also a significant focus on the differences between Ghana and the United States.
When he is nineteen, Guy acquires a girlfriend who is thirty-six years old. This upsets his mother, but she isn't sure what to do about it. Whether the relationship is explicitly sexual or not is never mentioned outright in the story. The two are evidently a "couple," but there is no particular allusion to the idea of them expressing their relationship sexually. The narrator does mention at one point that it is her son's developing sexuality which has led to the change in their relationship, but this is more in connection to the fact that he is growing up, maturing, and can no longer be seen as a child. Her interest in the relationship between her son and this woman is more about how it affects the mother/son connection.
There are some allusions to sexuality elsewhere in the book, but they are very brief and not at all explicit. Angelou refers to having "experienced the thundering pleasure of sex" when young in Georgia, and there is some (very brief) mention of the sexual freedom Ghanaian women afforded to their men.