They were teenagers once too, but they grew up. Even if they do put themselves in your shoes, it certainly doesn't mean they have to act like a teenager to do it. They're your parents, not your friends. They probably realize better than you do that whatever the issue is probably isn't a big deal, or they don't want you to make the same mistakes they did.

It's the same way you scold at a child for doing something wrong, even though you know they're just kids and you were like that once too. It's not delirious, it's an effort to teach them something.

Scaredycrow said it perfectly. They are only trying to make you better people than them. That's what every parent wants. It may seem unfair, irrational, or just plain stupid sometimes. But they only do it because they love you (or in this case your friend's parents loving him). They were once just like you and know the mistakes they made; they want to correct those mistakes before they are even made. If parents didn't give a shit about their kids, they wouldn't yell or discipline them. The fact that they do it shows they care, so you or your friend should appreciate that and not take it for granted, because they want to help you through life. Some kids don't get that privilege.

You'll understand when your a parent.
Think about it, I'm 15, and if my parents new that my boyfriends and other friends were 20 odd i wouldn't be allowed out the house.
They will tell you stories of when they were younger and you might think why can't I do that then?
I think that all the time, but you have to understand that the way the world works has changed, things that dodnt matter so much are of huge importnace now.
My mum used to skip school to see her boyfriends because no body bothered about it. but now you can get in serious trouble if you get caught.
Really, your parents are only looking out for you but because they are your parents you don't want to admit to it.
Honestly I know that some of their rules may be ridiculous, but to be fair for as long as you live in their house and they pay for your stuff i think you should have the fairness to treat them with respect and go by there rules at least for the most part X