How old is your sister?

I think that many of us would agree that a relationship between, say, a 31-year-old and a 23-year-old could be appropriate. At those ages, both parties are in a similar stages of life: completed schooling and setting out into the working world. At your age, though, you are still growing up, forming, developing, maturing, and in school. You are in a far different stage of life that a 23-year-old is, and of course a 15-year-old is not in a position to be speaking of marriage.