Currently, the information on young-adult reads:
This tag should be used for questions about fiction that is intended for readers approximately aged between 16 and 25. A common abbreviation is YA.
This is incorrect. Young adult books are aimed at teenagers. Most sources give the age range as 12-18, though some are different by a year in either direction. In no case is the age range anything near 16-25. (Note: anyone of any age can (and does) read YA books but the age range here is the industry standard.)
There is also a recent category called New Adult, for ages 18-25. I would have no objections to adding a tag for this, if it seems useful.
I have enough rep to change the tag unilaterally, but I'm posting here first to make sure no one objects. If I don't get any objections in a couple days, I'll go ahead and make the change. I'll also update the info on children and middle-grade to point people to the correct tags by age.