It doesn't matter how old you are, or how emotionally mature, when a parent dies it has a huge impact on your life.
I recall sitting at my mother's funeral, surrounded by a loving husband and children as well as wonderful extended family, feeling bereft. My mother was close to 90 and had Alzheimers. She was ready to leave this life. Yet the thought "Now I'm an orphan" kept ringing through my head. I was in my 40s at the time! Even as it was happening I could see the funny side of my emotional response under these circumstances, but it was a very real and raw response at the time.
In recent years I've supported my husband through the death of both parents, and seen the loss in his eyes. Yet when his father died he wouldn't discuss the loss, even though it was obvious to me just how deeply it had affected him. Just recently I contacted an old friend with my condolences on the death of his mother, a strong woman who'd been the backbone of the family. "It's been a tough day but it was her time to go" was his stoic reply.
We all know that death comes eventually to us all and if you survive to an old age you have been one of the lucky ones. But still we, in our Western culture, don't really know how to grieve the loss of our parents when we are older. Our Western traditions of death suggest that we should celebrate the lives of loved ones, particularly when they are elderly. But when your age reminds you that you are an "older person", but inside you feel like a distressed and lonely 5 year old, how are you supposed to act?
You may be interested to read an interesting article on this subject "How are baby boomers handling the death of their parents" recently published in the Huffington Post.