Why we write YA

So I was reading Amna's post  and got to rambling in her comments. The gist of the post was about whether or not our teen years are the best of our lives. Which got me to thinking why so many of us write YA, and read YA, regardless of our age. Now, I have a cold, and am running on strictly cold medicine and coffee while my college homework steamrollers my social/writing life, so this may not make much sense.

I'm 20, so I'm technically not a teen anymore, though I don't want to call myself an adult. Then again, if your characters are no longer in high school, it's hard to categorize the novel as YA.
Being a junior in college, I guess I'm not a young adult anymore. I did graduate high school at 17 and start my freshman year of college at 17 because I have a December birthday, so I was a teen, out of high school.
Why do we limit YA to high school? And why is that the genre we want to read? Of course, in the long run, I won't have an answer for this, but it's interesting anyway.

High school is a time of great change, sure. It's about finding yourself, finding your friends, falling in love. But I think you find who you really are in college. You don't spend 6 hours a day with your friends anymore, so you lose touch with a lot of them. It's tragic, it really is, but maybe that's how it has to go. You'll probably fall in and out of love a few more times, then find a real relationship. You'll pick a major, realize you never want to do that as a career, change your major, realize you never want to do that, have a crisis, finally pick the right major, and move forward. You'll discover what you want to do in life, who you always want to be around, who you want to live with, love, marry etc. etc.

It's the grown up world and it's terrifying, but once you're there, it's easy as pie. Like I said, I'm still only 20. College isn't the 'real world' but it's still this huge obstacle people fear. And you know what? It's not that bad. It's not even that different. Maybe I'm in for a huge shock when I graduate, but my boyfriend and a lot of my close friends have, and nothing is different yet.
There's a great chance I'm naive, but I like to dream. Even though getting here was easier than I dreamt it would be, I don't want it to end. I like having this wide openess still in front of me. I like the idea that I'm young, this is the golden time, I can do what I want and mostly get away with it.

But remember, picking a major doesn't mean that's your life career. Graduating and starting a career doesn't mean your locked into it. Never forget to dream. That's why we all write YA, right? Cause it's about dreaming, changing, and falling in love.
But people of all ages read it and write it. So why can't we live it?