13 years old

[TranquillityBase]

Mom was at the kitchen sink, sweating
and snipping beans. Her lips pursed,
then she asked you where you'd been.

You'd wandered off again, down the lane
the tractors made as they rolled through the fields.
You'd followed their tracks to the hill
where the ferns and the trillium grew,
checked the ground for poison ivy,
then sat down and wondered why.

Why your palms were still moving.
Why your head swayed without wind.
Why you had goosebumps in the middle of summer.
Why shade hurt your eyes.

The trillium bloomed every year in May,
but white to purple took only a week—
a floral Brigadoon.                                         This stanza seems a digression that doesn't add anything to the narrative. Maybe it parallels the child's transformation.  If so, I'd at least cut "a floral Brigadoon"

The tall corn covered you from the sun
on the way back, like a boyfriend drapes            move this to beginning of stanza
his arms around a girl.
The gentle scrapes of their rough leaves
felt real as beds and dinner plates.

The door to the kitchen opened itself;
another person's arm pulled the handle.

How to tell her that she didn't have a child?         this prepares us for that last stanza
She'd steamed the life out of those beans.

She didn't care where you'd been
or whether your hands were real,
only that they could shell a bushel of peas.          great final stanza

This leaves me, as a reader, wondering what exactly happened on that hill.  That's perhaps a good thing. You don't try to explain the transformation.  It's up to the reader to come to their own conclusions. The reader has a strong picture of the mother's viewpoint, but the child's transformation is left uncertain.  I didn't find it frustrating, just wondering why, as the child is wondering why upon the hill.

I don't understand the meaning of the title, unless it's referring to the child's age.  You might make that clearer.

TqB