1. Depends. If it's gratuitous and doesn't serve a purpose in the story, then it's too much. However, if it serves a purpose, then write whatever suits the story. I've seen pretty much all sexual experiences covered in YA; some pretty graphically, with named body parts and lots of information about what the characters respond to, some more tasteful. Some has been bordering on 50 Shades. Teens have sex and publishers and agents accept that. Just make sure it fits the story, and, I'd suggest if an author is not confident about writing it, veering on the side of caution/fade to black is best.

2. Your readers could be any age, but the typical age for YA protags engaging in such activities is 16-17.

3. I would have been as a teen, and so I imagine are many others. However, if your MC is over 18, it's going to be tougher to pitch your book as YA. That's just the way the market works.

4. What Sage said. I'd also add that even if the MC is 16 and the LI is, say, 20, it'll be an easier sell if the LI is aged down to 18, unless there's some really important reason for them to be 20.