I think about Life Experience (with capitals, yes) more than I probably should. This is the trouble with being a writer. You're always thinking about stuff, rather than actually doing anything.
It worries me that young people (young writers specifically because young writers are my people) are consistently being told that their opinions are invalid, or, in the case of young writers, that they cannot write, and that their stories are not of value because of their youth. That they need Life Experience. And this assumes a great deal about young people, and doesn't take into consideration the extraordinary diversity of human experiences. I think as a rule people assume to much about other people's lives. Who is to say young people don't have valuable stories to tell and opinions to share?
I think young people having respect for adults and knowing that they still have a lot to learn is incredibly, incredibly important, but I don't think learning from your elders and being able to share your own opinions are mutually exclusive. I think we build a more respectful society on the whole if respect goes both ways. I think we should be encouraging young people to think critically, and to express themselves creatively.
I'm not entirely sure why this discouragement is so prevalent - I think people have this concept that if teenagers aren't constantly reminded that they are young and stupid, we'll all grow up arrogant and self-obsessed. Humans are arrogant and self-obsessed as a species, and all we can do is our best on an individual level. I think every generation of teenagers gets a reputation as the worst generation yet, and we don't need to all get so panicked about it. Putting teenagers down all the time only really makes them feel misunderstood. There is no point to discouraging creativity and imagination.
You are alive, as you read this. (I assume. Good on you, if you're a ghost who's managing to operate a computer.) You are experiencing things. Life does not automatically start randomly at the age of 18, or 30, or 65. People who are older and have had fascinating and varied experiences are not by default brilliant writers (though they would certainly have a lot of material if they chose to take it up). People can be young and have had fascinating and varied experiences already. Even if you haven't had fascinating and varied experiences, you can still write. You still have imagination. You still have a capacity for empathy. You can still learn about the world.
If you're going to write for young adults, being a young adult is an advantage. You're experiencing the things you want to write about right now. Isn't that brilliant? You know how teenagers speak and feel and think, and what concerns them. You don't have to creepily lurk outside schools and eavesdrop on conversations (I sincerely hope no actual adult YA writers do this).
If you want to write for adults, or you want to write non-fiction, why not try that? One of the many wonderful things about writing, particularly of the creative and fictional variety, is that it relies on you empathising with other people. You don't have to be old to do that. Probably children are better at empathy than anyone.
I think you can be young and write and share your opinions and be open to growth and learning. Know that your opinions may change, and that you will mature as a writer and a human. But there is no arbitrary shift between teenager and adult, I don't think. There is no point at which the Life Experience bar is entirely full and a little bell goes off and announces 'you are ready to write!' If you want to write, start now. If you deliberately set out in life in search of Experience so you can write, I don't think you will find it. The more you live, the more material you will have to write about, the more your ideas and concepts of the world, other people and yourself will form. But you don't have to wait to start writing. You will always be learning and growing and changing throughout your life (I should hope. Ask me again in fifty years). You can write and live your life at the same time, honest.