1. I generally have the same criteria of liking a story even if the protagonist is a woman which is: 1) Good character. Nothing one dimensional or boring. 2) Doesn't make me upset/angry. What I mean is, someone who's sole purpose is to be as mean as possible with no regard for others. 3) A good plot centered around them. I appreciate if they actually do something worthwhile in the story and not, for example, just be a character who only wants one thing(In sports shows/movies it's a title or whatever, in romance it's a girlfriend/boyfriend, etc).
2. Depends. How old are we talking about? You don't want someone so much older than you that you can't connect in some way but you also don't always want to be writing about young people all the time.
Overall, guys(and girls) don't usually mind a story where the opposite gender is the protagonist. If it's a good story or good plot overall as a whole people don't care. For example, growing up I used to watch the Teen Titans a lot. There were five main characters, 3 boys and 2 girls. One of the boys I didn't really care for but the rest of the team I did. It didn't matter if they were older than me or girls, I just liked them because they were funny and had nice action scenes.
Now in some circumstances, yeah, gender is a problem. Some romance stories on this site have girls as their main protagonist and when they get to romance scenes and such I'm just like, "........really? The point of all this is......?"
I was bored. Really bored and not interested in the story or character, even if it was good.
In most cases though, if it's not heavily on romance, gender really isn't a problem I feel. Hope this helped.