DD1 was 21 months and was at the birth for DD2. It was very nice. I did have my mother and my sister there to help her, and I needed to her to be out of the room at one point so I could get out of my mommy head and really focus on birthing. We watched UC and homebirth youtube videos before hand and talked about how being noisy helps some mama's get the baby out, and it's hard work for a mama to push the baby out of her tummy.
She asked to be there for this birth again.
It would have been incredibly upsetting to her to be sent away, and then to have her come back and find this baby in her home. VERY upsetting. Instead, she hopped into the birth pool with us and was cuddling right next to me as I latched her little sister on for the first time.
But then, every kid is different, and maybe for another child it would be more upsetting to be there, but I knew that wasn't the case for my daughter.