Zippia? McD claims that the average age of an hourly employee is 20. Presumably we are talking about the cooks and cashiers earning an hourly rate.
https://www.mcdonalds.com/gb/en-gb/help/faq/18338-what-is-the-average-age-of-a-mcdonalds-employee.html
But if you're in your late 20's or older and still working the kitchen and drive through at McDonald's, that's your own fault.
Unless your only source of income to pay for college is working at McDonalds, which isn't paying enough for tuition....
Well... it would benefit everyone in society so... I mean... not just personal.
I can afford insurance just fine. It's not going to benefit me if the government increases taxes to support people to supplement the lives of twenty year-olds with temp jobs and people who failed in life with this new massive socialized healthcare system.
You want to take from me and provide nothing in return? What a compelling proposal. 
First off, as you've said, most people get insurance through their jobs. So technially, you're not affording it either: your employer is. If you lost it right now, how much would your plan cost out of pocket?
And you'd get the national health care plan as well. So you would lose the payment to your employer (unless they pay for it 100%) and still have insruance. You'd also lose Medicare and Medicaid taxes on your paycheck. Probably merging them into one payment, which would benefit you directly since you'd get the insurance your taxes are paying for.
Your employer could then take all the money they save from not having to pay your insurance premiums every month and give it to you as a pay increase.
And the best part?
Your taxes don't have to go up. Know how much I pay in taxes? 34%. My big, bad, evil national healthcare system isn't even a separate tax. Its just part of the 34% income tax I pay on all my income. But I guess none of that benefits you, huh?
I've seen this argument before.
Ok, tell me something: What is the progression of career that you think happens? What kind of economic progress do you assume is the normal?
I think it's normal for people to pick up menial jobs early in life and work to make something better of themselves.
How expectedly vague.
So, how do they better themselves? School? Because last I checked, college isn't free. Ya need money. Which most people get by working a job. Oh wait.... the job doesn't pay enough for the hours you can work after school. (Been there, done that. And I went to community college).
See Tom, I have ACTUAL experience with this. Do you? Have you ever worked at McDonalds or some service job? Ever had to make car payments on a used car, insurance, and tuition to a community college on a McDonalds Salary? (not to mention gas driving to work and school) Because I bet you haven't.
And before you cry "Scholarships!" remember: Of the hundreds of thousands of people who may be applying... only a few get it. So odds are, you're not getting it.
"Loans!" you say to counter: Yeah. Great. Oh wait no... you need to prove you can pay it back WHILE IN SCHOOL. Yeah, good luck with that on a McDonalds salary.
Or, you know, have parents who will thrown down their house as collateral.
See, maybe, just maybe, having a decent wage would give these "young people" capital they can save and invest in education so they can give themselves a good jump start on their career while having no experience or job skills yet. Instead of, you know, being a burden on their parents even after they get a job.
Also: Why is a job, that is needed, for "young people"? You do know that McDonalds didn't start their business to give young people a job, right? They started it to sell burgers. And if young people don't take the job (as seen by the OP), the job doesn't get done(because older people can't love off the wages) and the restaurant closes. So... why do you think its for "young people"? Because the way I see it, its not FOR young people, but rather the only jobs(expanding to the service industry as a whole) young people are qualified to perform.
McD is a business and doesn't care if its workers are young or old. The workers trend younger because it is a norm to work menial unskilled jobs early in your career as you try to make something of yourself. McD managers know it's just a temp job. The workers know that it's just a temp job. I know that it's just a temp job. I'm not sure what your deal is.
If low paying unskilled jobs didn't exist then those young people would probably be leaching off of their parents even more than they are. It serves a purpose which the parents appreciate.
1. McD managers were employees so... not a temp job for them.
2. You have it backwards. You assume that they take these jobs because they want them. Trust me, we do not. We take them because we can't get anything else. Because being 18 with no education beyond High School means you need a job that has no skills. Its not "Norm to work menial unskilled jobs" it's literally "the only job we can get". Its not a cultural thing, it's a literal "We can't do any other jobs yet".
If low paying unskilled jobs didn't exist, we wouldn't have McDonalds, dumbass. Or movie theaters. Or restaurants. Or bars. Or department stores. Or grocery stores. Or any other business that relies on unskilled jobs. Hell, we wouldn't have fresh picked strawberries either.
These aren't jobs given out of some pity. They are essential for whatever business venture needs them. Every job is important. Go to McDonalds and try to make your own Big Mac and Fries. See how that goes. Or head to Macy's and see how easy it is to shop if no one has stocked the shelves.
These jobs exist because they are needed, because something needs to be done. It may not require great skill, but it requires a human. And that human is worth being paid enough to actually be able to live until they can move on in their career.