The garden near Esperanza's house comes into play at a significant time of her life. She's caught in that awkward period of adolescence where she still wants to act like a little kid, but she's also starting to think about grown-up things like sex. Sounds like she's poised for a loss-of-innocence moment in grand Garden of Eden style.
The archetypal garden, like the Biblical Garden of Eden in the book of Genesis, is a place where it's always springtime, people live in harmony with nature, and everyone runs around naked without being embarrassed about it. Life is generally awesome. And that's pretty much what the kids on Mango Street find in the monkey garden (minus the naked part, because Chicago has a shortage of fig leaves). At first, when the neighborhood children take over the garden, it's "a wonderful thing to look at in the spring," full of flowers and fruit trees (38.4). The garden becomes their refuge from the prying eyes of adults, a place where they can play their games and build "no grown-ups allowed" clubhouses. They even start a rumor "that the monkey garden had been there before anything," heightening its Eden-esque qualities.
Which brings us to the second half of the garden-as-Eden archetype – the part about The Fall. (You know, where Adam and Eve eat the forbidden fruit, realize the fig leaves aren't really hiding anything, start wearing clothes, and get kicked out of Eden.) The monkey garden slowly becomes corrupted, presaging the fall from innocence of its virginal occupants. Weeds start to grow in its flowerbeds, and it fills up with dead cars.
So it's no surprise when Esperanza, who wants to play games with the kids even though someone says she's getting too old, figures out something about sex when she watches Sally go off in the garden to kiss Tito and the boys. A talking snake may as well have given her an apple.
Of course, Esperanza doesn't adjust well to this sudden revelation. And afterwards, "the garden that had been such a good place to play" doesn't seem to belong to her anymore (38.24).