I read this and didn't comment but, I'm going to say it:
I've worked at teaching hospitals a lot, so I understand your frustration. In the student's defense though, I have to say, I think it's the instructor you should be upset with. That's just me though.
I see it as, while the instructor had great hindsight about the student's behavior (hindsight is 20/20, but I'll give her that she apologized for the situation after the fact), that's not where their input was needed. Truth is, while on the unit for clinicals, students have to report everything to the instructor. So, the constant reporting to them was not an issue from my point of view.
So, we have a student who is questioning all your advice and guidance. They run to the instructor with far reaching attempts to discredit you. Why did the instructor not correct it while it was going on?
I say this was not an issue of Y-ports, pain management or potassium. It was a bad attempt at belittling you. The issue was this need to appear superior to you, not the details (some, rather obvious details at that) of the care. The instructor should have seen that after so many attempts were made to find fault with your viewpoints.
Just my two cents but it had nothing to do with your age. If the student hadn't been using your age as their rational for their behavior, it would have been that you took too long in report, or you were talking too fast, or you didn't dress professionally, or............or.............or. Point is, he just picked your age to critique because it was the most convenient stick to pick up and beat you with. If it weren't that, they'd have found something else to bad mouth.
That student is passive aggressive, as evidenced by the fact that they were gossiping after the fact in the parking lot instead of making their feelings known at the time. If your age bothered them so much, they should have dealt with it and gone to the instructor about it then. Don't be a coward and complain well after the fact. Since they didn't do that, I can only surmise that it didn't really bother them that much.
This student is going to continue to be passive aggressive like this until someone calls them out on the floor for it. The instructor should have assigned them an article to write about the difference between passive aggressive and assertive, with being allowed to attend clinicals on hold until said article is written to satisfaction. THEN I'd believe the instructor's apology.