for Q1, girls like older guys because girls like to be taken care of. they love someone can teach them things and solve problem for them and this kind of guys normally are older than girls a lot. Thus, if a guy can protect, take care of a girl, his age isn't a problem , definitely not!
for Q2, guys like younger girls, because they think that they can fully control those young girls, like those young girls tend to be satisfied by a small amount of money or a little , tiny gifts (also, coz they are too young, they may even not know how to say no)
for Q3, at the same age , girls are more mature than guys, thus, it is hard to achieve agreement when they are discussing an issue, like marriage, career, family, study and so on. Thus, generally speaking, it is relative hard for a young couple to stay together peacefully.