I and my two older sisters saw the birth of my littlest sister when I was 12. It was pretty amazing. I don't know quite why my mother let us watch--perhaps we just wore her down with nagging!--but I'm glad she did, especially since Abby turned out to be the youngest member of our family.
Details have sadly blurred over the years, but I remember Abby crowning... watching the contraction 'numbers' jump on the machine and saying 'That was a big one, Mum!' (yes, I'm sure she needed to be told!)... a fair amount of blood and goop and gore, and not being perturbed by it... being very impressed with how calm and rational Mum was during the event (she warned us all beforehand that if she yelled at us, it was just because of the pain, but she never did yell at all)... Dad fainting... my older, medically-inclined sister who'd been snobbish about the whole thing coming over all funny when Abby was born and having to sit down... me anxiously telling the midwife 'Don't drop her!' because Abby looked so slippery. And of course, the immense and overwhelming relief that the baby was a girl--my family had 5 girls already, but my sisters and I wanted to keep it girls-only!
Now I'm pregnant with my own, I can't imagine having ANYONE other than DH and the midwives present at 'my' birth. Maybe that feeling will change when I have older children, but for now I'm being jealously exclusive--selfish me!