I got back yesterday from a trip to Chicago to bury my grandmother (my dad's mom) who died a few days before. She was almost 92. The trip was sad, of course, but it's always nice to see the family (well, most of the family anyway). I've been very lucky to have had 4 living grandparents until the remarkably old age of 34. Now I'm almost 39, and I still have 2. My dad's dad died first about 4 years ago.
My wife was musing about how undemanding the role of a grandchild is. Grandchildren (most of the time) don't have to be anything but kids for their grandparents. The responsibilities are all in the relationship with your parents, not with your grandparents. I've heard the bit of wisdom "Most people get along great with their grandparents. The reason? You have a common enemy." Which overstates it a little, but you get the idea.
And so the loss of a grandparent is a very real loss of a part of your childhood. My wife is good about pinpointing the source of feelings like that.
There's another reason it feels like a loss of childhood, too. When you're young, if you're lucky enough to have living grandparents, you are in the 3rd generation of your family. There are two generations above you to protect you. From monsters, unpleasant family history, starvation, and other real or imagined threats. On my father's side of the family (My mother's parents are still doing okay at 91.), there's now only one generation above me: my dad and his sister. And sometime in the next year (grrrrr....more and more waiting) I will have my own child. That feeling of protection is disappearing. Like I'm being forced to vacate my safe ground-level apartment to a second-story apartment that's more prone to tornados, falling satellites, and tumbles off the balcony. The unprotected third story is just that much closer.