So speaking of age difference between couples...what is considered to be the borderline between "should be less" and "that's just wrong!" ? is 10 years "somehow acceptable" but 15 years "way outta line" ?

My girlfriend is 2 years younger than me. No story there. I've got a friend who's 26 and his girlfriend 20. Again, no story.
But I've got another friend who's 32 and his girlfriend 23. There's more than 9 years of difference between them, or you can say there's a decade between them. Now they love each other very much, live together, and I wish for everyone to be as happy as they are right now. The girl's mom seems to be ok with it. She's happy that the boy is "settled" and has a steady job, his own apartment etc. He's also a very nice guy, btw. But the father is not so happy. He keeps telling his girl that when she'll be 30 then he'll be 40, and then she might start to regret her decision. And I'm just wondering what's the borderline here. I'm pretty sure that if it was 6 or 7 years of difference the father would not have said anything, but 9 years seem to bother him.

I guess 15 years of difference is when people start saying stuff like "he could practically be her father". I suppose if someone had his first daughter when he was 20, and she went on to marry a guy who's 35 while she was 20, then yeah there's only like 5 years of differnce between the father and the husband. But then there will always be people who say that as long as 2 people love each other, age shouldnt bother. But it's not always so simple...

Another thing I'd like to discuss is the fact that the man is usually the older one. What if it was the other way around and the boy was dating an older woman? is it then even more of an issue? should the boy's parents give him hell about dating an older woman, say 10 yrs older than him?

I'm also interested to know how age difference is perceived in other cultures. Maybe svsg could tell us a little about India, afsaneh about Iran, etc.