My son was in the delivery room when my daughter was born. He was 8 years old and did GREAT! My husband held him up towards my head so he was looking down at her being born. When she came out, he immediately buried his face in my husbands chest and said, "I already love her" and was crying with joy! It was one of the most amazing experiences for my son and us.
Also, we had gifts ready to give to my son from the baby. He is now 17 and still has his bear in his room!