I just watched it again and I noticed that too, she looked like a 30 year old waitress.

You're right it's the slim waitress with the straighter and brighter blonde hair who puts something in the coffee, serves it, and helps with killing the one man. She's definitely not middle aged, her arms and built aren't those of a typical middle-aged woman, and you can also see her face twice (for just a brief moment) and to me she looks like she's in her 20s, so its possible that she's supposed to one of the older teenagers.
But it becomes confusing when we see them leaving after the deed is done, as there is clearly the chubbier waitress with the darker and curly hair moving towards the door. Her figure and hair style make her look like a middle-aged women but we never get to really see her face so it's hard to tell how old she is. So maybe she too is supposed to be a teenager?

I always thought the waitress who poisoned the coffee was Rachel, the girl who is later shown conducting the Passage ritual at the church, when Burt interrupts the proceedings.

My memory was fuzzy until I just saw the opening scene on youtube. There were two waitresses, the one being talked about in this thread, and another one with lighter blonde hair. The second one, the one with the lighter blonde hair and her hair was not as curly as the first one plus she had her hair in a ponytail, helped to kill the man who apparently ran the diner.

I didn't understand that either. I assume maybe she was an older teenager, like eighteen? I think maybe it was a deal she made with the children as well. The mechanic was someone they had made a deal with as well to lure the adults away from Gatlin. So I think they may have persuaded the middle-aged woman to do their deeds.
I get high on hydroponic weed

To answer the original question: I think they're referring to the waitress who was pouring the poison powder into the coffee pot. I didn't really see her face directly, but from the back, she appeared to be a middle-aged woman.
If I'm not mistaken, as the killers were exiting the Diner, they showed her from the back and she was wearing a blue waitress uniform / dress over a pair of black pants or something. That really stood out for me as odd, like she just put on an extra waitress uniform that the found in the backroom of the diner and she didn't even work there.
But then those loyal Sunday customers would have noticed. Anyway, she seemed to be a lady in here 40's easily, from the first time I watched this movie. I didn't understand this either.

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True, but that's because he has a genetic disorder.

John Franklin who played Isaac is only three years younger than Linda Hamilton & six years younger than Peter Horton, he was 24 years old during filming playing a 12 years old, that reminds me Ralph Macchio in "The Outsiders" second older of the bunch (only Patrick Swayze was older than him) & he was playing the youngest.

HaHa! Near the end, a kid standing in the circle in the cornfield looks like he's 30.

She wasn't old, she just had big 80's hair that makes her appear older.

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I always just thought she was a teenager. I did notice she was older than the others but no older looking than Malachai.

I know who you mean, she's just an older-looking "kid". As someone else in this thread mentioned, there's another one of them that looks like he's 35.
I'm happiestin the saddle.

She might have lived in another town and the kids let her live for helping them? Not sure the true answer but just a thought
Nope,I can't go to Hell,Satan has a restraining order against me

I'm sure. She's exiting along with the other kids. Also, she's carrying a meat cleaver.

Are you sure she's a killer and not being killed? xD