For me, its been different with each birth. Each time my kids have said without a doubt that they WANT to be there though. It'd be different if they didn't want to.
My ds was asleep in the room nextdoor when I had the surro babe, my dd was in the room but only 18 months old so I don't think she understood anything.
With Pai, they were asleep but my ds was woken up by my noisiness. I had decided not to wake them up, but since he woke up on his own, it was fine. He ended up standing just inside the door-he didn't want to watch THAT closely, but still wanted to be there. We woke up his sister immediately afterwards and she got to see her right away too. It was nice
They all wanted to be there for my youngest as well but I ended up having no one there (surprise cesarean at 33 weeks). Not that they could've been in the room with a cesarean anyway
Anywho, they very much want to be there again, so barring any problems, they will be. My ds is 9, will be closer to 10 by then, but he still wants to be there. I think it helps that he's A.) been there before and B.) I've watched hundreds of birth videos on my computer and they've all seen quite a few-I think that helps up the comfort level. I also would never force any of them to watch the baby come out. They can stand where they feel comfortable. They know where the baby comes out and what happens. At the same time though, I'm not a shy person and while I don't parade around naked, my kids do see me naked from time to time-mainly in the tub or after a shower when I forgot to get my clean clothes (lol). So the nudeness isn't a big deal either.
I would always leave it up to the child though. If they are very sure, I think they should be allowed to be there. If they are the opposite, they shouldn't. And if they are in the middle, perhaps a bit more education and talking is needed. But regardless, it should be the child's decision (provided mama is ok with them being there as well!)