Is it OK to expect more out of older students (behavior-wise)?

In one of my classes, I have a student who is much older than all the other students (6th grader vs. the 3rd graders). I often forget how young she actually is until she gets out of line, behavior-wise. That being said, I am wondering if it is fundamentally alright to ask her to set a good example for the younger students. Of course I don’t wish to single her out, but she is aware of the age-difference. What she doesn’t realize, or maybe does, is that the younger girls worship her and are quick to jump on her bandwagon of destruction. (She is actually a great student and extremely smart, and in most cases, well-behaved). As a kid, I hated it when my parents asked me to set a good example for my younger sibs, but I feel like this girl is old enough to understand? I don’t know. Is it OK (or fair) to remind a student that she is a role model to her younger classmates?

I usually have a few disruptions at the beginning of a week, then smooth sailing for the rest. Weekends do something to children.

At this point, I’ve just honed some discursive devices, translating when necessary.

Me: “How old are you?”
Student: “I’m n years old.”
Me: “Oh, so you’re not five years old.”
Student: “No!”
Me: “Are you sure? You seem like you’re five years old.”
Student: “No, I’m not!”
Me: “Then why do you P like a five-year-old?”

Or I ask them sardonic questions:

Me: “Could you, um, stop making us hate you? Thanks!”

And so on, until the student gets the idea that I think she’s acting inappropriately for her age.
I have the reverse problems with girls, though. I simply can’t hear them when they read.

It is fine to expect better behavior out of older students. That does not necessarily mean you are going to get it. I have had adults who have given me problems.

You can communicate those expectations in a positive way through extra responsibilities, praise, leadership opportunities, attention from you, etc.