365 Days.

There are 365.25 days in a year, no matter what year it is.
That being said, why is it that two years' difference in age between two people goes from trivial to massive once the older of the two graduates from highschool?
My mother had a panic-attack because I was supposed to go on a date with a 19 year-old boy. I'm 17. She had no objections last year when I spoke of a senior that I'd been hanging out with. So why now? What makes the big difference?
Her only argument is that he's graduated from highschool. So the question becomes, what is it about a diploma that makes someone so incredibly mature that a mother needs to worry about it? There is no valid argument to this. None I've been presented with, anyway.
My sister dated a senior. While the relationship only lasted six days, my sister is a freshman and my mother had zero objection to this. Granted, the situation was a little different because she'd known the senior through me prior to their dating, but do you see my point?
There's some ridiculous idea that a highschool diploma is truly the difference between a child and an adult. Why? It's insanity. I can't wrap my mind around it, and it makes me angry.