This poem begins with a metaphor comparing childhood to a place where nobody dies. In the innocence of childhood, nobody really does die, the author says. Oh, sure, old people die, and maybe your cat will die, but not important people - not people that matter - certainly not your parents.
The visual image I get from reading this poem is that the author is a grown woman sitting at the table with her elderly mother, drinking tea:
To be grown up is to sit at the table with people who have died,
who neither listen nor speak;
Who do not drink their tea, though they always said
Tea was such a comfort
These lines illustrate that the speaker is now grown up, but she is thinking back about how easier it was as a child, when she didn't realize that people DO die, even parents. Now, she is sitting at the table with an older person who appears to be rather senile, who does not pay attention to her tea -- who does not really pay attention to anything:
Shout at them, get red in the face, rise,
Drag them up out of their chairs by their stiff shoulders and shake
them and yell at them;
They are not startled, they are not even embarrassed; they slide
back into their chairs.
The speaker seems to be frustrated with the elderly person - her mother, perhaps - and longing for a time when she did not have to face the reality that her mother is old and won't be around for long.
Does this help? Go over the poem and look for the language that creates a melancholy mood and see if you agree with me.