Me, why shouldn't one do that? But my last resume is 10 years old, maybe I am out of touch with all the mumbo jumbo dancing you have to do, to build the "right" resume.
Well you can't make a hiring decision on that basis in most places unless you have a reason. What constitutes "a reason" being variable. Generally if you are prohibited from making a decision on a certain factor, you may not ask about it during an interview.
Sex discrimination can be constituted by various things. For example asking about maritial status, children, plans for pregnancy, soliciting sexual favors, etc. Also in some places, if you thought someone might be trans, you could not ask them about that.
Some of us don't clutter up our resumes with every job we've ever had. My resume lists nothing irrelevant to my current career. I was well into adulthood at that time. Who cares where I went to highschool? It demonstrates respect for the time of the person tasked with reading a stack of resumes to not waste their time.
People probably don't care where you went to high school, but they probably care if you have a degree and when you got it. Most people go to university within a few years of finishing high school.
According to US statistics, "overall college enrollment rate of 18- to 24-year-olds (ages in which students traditionally enroll in college) was 38 percent in 2021".
So if by "most people" you mean, "less than half" then yes you are correct.
Most people who have a higher education degree. I thought that was implied as you even mentioned it in your comment "ages in which students traditionally enroll in college".
Unless you are trying to get a job with min/max age requirements, like airline pilot or us president, age provides no valuable information to potential employer other than a factor to illegally discriminate on.
How is it relevant? Why should an employer care about your? All they should care about if you can do your job or not. It's also illegal in many places around the world to make a decision based on someones age.