In households where the adults read LOTS the children are reading before they start school.
I have to disagree that where adults are avid readers the children will inevitably be reading before they start school.
Children are individuals, and they do not always follow their parents example, in reading or anything else. Two of my children became avid readers, two almost never read at all. They are now all of them adults, who grew up with parents who rarely let a day at home pass without spending a few hours of it reading. I read to them a great deal before they went to school. They knew that for their parents reading was among the greatest delights in life. Still, they decided it was not for them.
When my son was seven, he decided he wanted to join Little League. I thought, "Baseball? Why would any child of mine want to play sports? He should be home reading a book." But he didn't enjoy reading, although he did it very well. So I thought about the fact that there were children whose parents loved sports and thought reading was a waste of a boy's time. I decided I didn't want to be like those parents, imposing my own tastes on my child. (And I hoped I was putting it out into the universe that somewhere, to balance it out, a child whose parents wanted him to play sports would give in and let him spend his days with his nose in a book instead.)
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But as to the question of the age of YA protagonists. Young people who read a great deal will read books
about adults written
for adults. They have no problem with that. So there is no point writing a book with an older protagonist and calling it YA. If they want an older protagonist they will find one elsewhere. They probably read about characters of all ages, and are perfectly happy doing so. But readers who are specifically looking for books about protagonists roughly their same age facing roughly the same kind of problems they encounter or expect to be encountering soon are not going to be interested in reading about adult characters (18 and up) facing adult problems. These days, whether to be sexually active is one of those problems kids face. Not so much in my day.
Write the book. Write it as the story needs to be written. Don't try to fit it into a category where it doesn't fit, and don't try to justify calling it something it is not.
If you truly want to write YA, familiarize yourself with the genre. (And by that, I mean
read a great deal of it.) You may discover that it is not for you. If it is, then stories and characters that fit within the broad outlines of YA will occur to you. If it isn't, why worry about it? Write the books you want to write. Figure out later where they fit.